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But beyond the implications for car sharing as a business, the news is an important indicator of the global and infectious trend of sharing stuff and resources. Airbnb — home sharing — is now looking at a $2.5 billion valuation. With 9 billion people set to live on the planet by 2050, and much of that growth happening in cities, sharing — enabled by IT — will be one of the most important tools to emerge. The companies that are creating massive brands now will be well positioned to capitalize on this future.

The little business that could

Car rental companies are indeed the original champions of cars as a service. Hertz at one point reportedly considered buying Zipcar, but instead launched its own service. Many of the major car rental companies are either testing, or have launched, modest car sharing trials in select cities.

But Avis’ acquisition is one of the largest financial bets to date on the undertapped potential of the more modern version of car sharing. The acquisition price was $12.25 per share, or a 49 percent premium over Zipcar’s stock closing price on Monday.

Zipcar has 760,000 members in 20 metro areas in the U.S., Canada and Europe — but that’s not really all that much considering Zipcar is more than ten years old. Modest estimates project as many as 7.5 million users for car sharing services (not just Zipcar) over the next few years in the U.S. if there are supportive policies in place, and up to 20 million in the maximum case scenario, according to an analysis from RAND Corporation.

The biggest problem for Zipcar was access to capital to help grow its user base. Maintaining and expanding a fleet of cars is expensive. Zipcar only first became profitable at the end of 2011, and its stock price has steadily declined (hovering around $8 most recently) since its IPO in 2011 — back then it had a high of $30 per share.

So in that respect, the Avis acquisition is great news for Zipcar and will provide it with deep pockets. The company will be able to grow more quickly in a sector that has seen increased competition for the rental car companies from new startups with innovative takes on car sharing (like Getaround), and even from some creative auto makers like Daimler.

However, as Felix Salmon points out Zipcar’s brand is really strong with its customers and is a well-liked company. Rental car companies, on the other hand, generally are disliked by their customers for things like inconvenient service, hidden fees and the like. The Avis brand could really taint Zipcar, especially if Avis starts to make Zipcar act like more of an old school rental car company.

Avis needs to just let Zipcar operate its business as it has done and not try to change the brand or the service too much. Avis can also use Zipcar as a model for making its rental car business even more distributed (away from the centralized car rental hubs at airports) and even more on-demand. The backbone of this car as a service model is IT, and Avis is acquiring the leader that invented all those easy to use reservation systems, mobile apps, and key fob locks.

A win for investors

The Zipcar acquisition also shows how some of Zipcar’s early investors made a very long-view bet on the trend of car sharing and mostly won that hand. Zipcar had raised at least $67 million in venture capital funding from Revolution Living, Benchmark Capital, Greylock Partners, Smedvig Capital and Globespan Capital Partners, reports Fortune’s Dan Primack (in his email Term Sheet). Benchmark led an early round of funding back in 2005. Revolution (Steve Case’s firm), Benchmark and Greylock will walk away with $150 million.

It’s one of the better exits for VCs that invested in transforming transportation. Most electric car and biofuel investments have struggled. But car sharing is based on using the web, mobile, computing and wireless tech to manage the on-demand use of cars, and that’s historically a more natural investment for a VC to make.

Still Zipcar returns have taken longer than, say, most web companies. Zipcar is almost 13 years old and went public at 11 years old. The multiple is also not all that high in the VC world.

It’s a sharing world

While Avis might not know it, the rental car company is stepping solidly into the global trend of sharing stuff. Some call this trend collaborative consumption, and others call it the mesh. Examples of these companies include everything from home sharing company Airbnb, to the emergence of co-working spaces, to baby clothes exchange site thredUp.

The easiest and most obviously shareable goods are big expenses that are underutilized and that can be managed with IT. So homes and cars are good examples — most cars sit idly throughout the day. Regular stuff that’s cheap and easy to own aren’t that sticky for sharing (I think the whole tool or neighborhood sharing movement has been over hyped).

The really great thing about car and home sharing is that it also has a sustainability slant. Car sharing reduces car ownership and makes the use of a single car much more efficient. It’s almost like the merger of public transportation and personal cars. Same thing for Airbnb — the more homes and apartments are shared, the fewer new ones get built.

Young, urban people all over the world will be embracing this trend. As cities get more cramped — and as world growth happens in cities — owning a car and finding a place to park it will become harder. Zipcar CEO Scott Griffith told us that it’s the Millennials (18-34 year olds) that will truly embrace car sharing. Millenials think “access is more important than ownership when it comes to transportation and car ownership,” said Griffith.

Betting on the habits of the next-generation of consumers is a pretty good place to be. And Avis — an old school rental car company — just got pretty good access to that new set of customers.

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And yet the ambition is there — and company he runs, online vacation rental company HouseTrip, is doing well right now. It just raised $40 million and is even pushing the much-vaunted Airbnb into second place in parts of Europe.

The businesses are not exact rivals — HouseTrip focuses on helping people rent entire homes for their breaks, rather than rooms — but they are both taking the continent seriously. There are a number of different ways you can compare the two companies, and in many cases Airbnb comes out on top: it’s certainly the biggest beast in some of the popular urban destinations around Europe, even when you strip out the room-only listings.

But at a national level, Housetrip is ahead in many countries across the continent — with far better coverage of rural locations and out-of-the-way hotspots that appeal to families on vacation, rather than urban travelers on short breaks. In its home market of France, for example, Housetrip emerges narrowly ahead in volume, while in other popular destinations (Spain, Croatia and others) it boasts a greater lead in its number of listings.

In a highly competitive market that includes rivals like Wimdu, OneFineStay and many, many more, that’s no mean feat.

And the fact that Bertrand cuts against the grain is, perhaps, part of the reason for that success.

The Insider

After all, unlike most of the people fighting for this market, he didn’t end up in the business by accident: he started out in the world of hotels. The idea for HouseTrip came to him and his wife, Junjun Chen, while they were studying at Ecole Hôtelière de Lausanne, the oldest and most famous hospitality school in the world.

“When I studied hospitality over five years I guess what you really understand is service — how to make people feel welcome,” he says.

“If you ask the team I’m very, very detail-oriented, and the product is all in the detail. What gave me that focus on the details, certainly, was studying hospitality. In Paris hotels, if one pillow is not perfect, or one knife is not exactly two centimeters from the edge of the table, it’s a big, big deal.”

Hidden underneath his crop of curly red hair, it seems, is a mind focused on getting it right and almost nothing else.

The company, which started in Switzerland but now has its HQ in London, became unique when it took on its latest funding. The round was led by Accel, making HouseTrip the only company that all three of Europe’s leading venture firms — Accel, Balderton and Index — have backed. They’re all on board because they see a massive opportunity, and (perhaps) a chance for Europe to exert its power over foreign competition.

This, says Bertrand, plays to the continent’s natural strengths.

“When you look at most of the big travel companies, they’re European-based,” he explains. “By far the biggest is Booking.com, which is a Dutch startup. Travel is different maybe because that’s our DNA in Europe: Most of the market is European; Most of the schemes for decades have been European; We invented the hospitality industry, and so on.”

He may be overstating the case a little (Booking.com is a powerhouse, but it was acquired by the Connecticut-based Priceline back in 2005) but Europe is definitely HouseTrip’s stronghold right now. Its biggest cities include Paris, Barcelona, London and Berlin, where demand is high and the company is working hard to keep its supply of listings closely matched.

But while Bertrand says those large locations are profitable — Paris, he claims, hit breakeven nearly two years ago — the company is taking on a lot of funding: near $60 million at last count. He is using venture money, he says, to try and achieve scale. This means opening up new destinations and bringing on fresh listings, which can be an expensive, arduous task.

“Destinations that are big and have reached critical mass are profitable, but we’re expanding to lots of new places at any one time and that’s what really costs the money — but that’s what gives us so much growth,” he says.

“We could decide tomorrow that we’re happy with the existing destinations we have and become much more profitable, but we still think there is huge room for growth, so we want to capture the opportunities.”

Focus, focus, focus

For now, however, that need to grow is tempered by the desire to keep quality up — his obsession with quality. That means that although cities outside Europe, like New York, are growing fast, the company is primarily sticking to what it knows best.

“I think Europe will still be our focus in the medium term. This market is a European market, and if you conquer Europe you can conquer the world.”

For all the drama and talk of competition between HouseTrip and Airbnb and the rest of them, though, it’s only a small part of a very large story. Holiday rental is a big industry — big enough to support a public company like Homeaway, which is currently valued at around $2 billion — but it’s dwarfed by the much larger hotel sector.

And long-term success may require a change in strategy for many of the players in the game. Like others in the collaborative consumption sector, I have spoken to, Bertrand believes that Airbnb is soon going to have to move sideways into some unexpected markets to maintain the growth its investors seek.

After all, there’s a certain kind of pressure that comes from the $220 million that Airbnb has now had pumped into it — maybe more soon, if rumors that Peter Thiel is interested in investing another $150 million pan out. Building a company that can match those expectations may mean expansion into areas like, say, car rental, or a deal with the hotel industry to offload empty rooms.

Bertrand doesn’t think the same sort of pressure will come to bear on HouseTrip, even with the bundles of cash it has taken. For a start, he says with a smile, his wife and co-founder Junjun is the CFO. But more importantly, his business fundamentals are more sound: renting entire homes means that the average booking on the site is higher than many of its rivals. And that, in turn, gives it a stronger position to attack new destinations, new places.

But more than that, it’s a philosophy.

“My whole vision from the beginning is that holiday rental is much better than hotels, more authentic, and a much better way to travel,” he says. “I wouldn’t be able to look at myself in the mirror.”

Still room at the inn

It’s clear that this game still has a long way to go, and the market in two or three years may look very different from today. The vast number of names currently in the mix are likely to consolidate, and perhaps one or two of them will get snapped up by the bigger travel giants.

Still, though, Bertrand says there’s a lot of headroom — even at the most basic levels.

“We’re still focusing on what we call internally the low hanging fruit — on the rental side, that’s people who have been renting out holiday rentals for a long time and are simply looking for a more efficient way of doing it. When you look at that market in Europe it’s 3 million properties, it’s massive and there’s a lot of room to grow in there.”

And how many of those 3 million properties would HouseTrip need to make Bertrand feel comfortable?

“Three million is the answer,” he says, without blinking.

Oh yeah. There’s that ambition again.

Update: This post has been edited and updated to clarify how Housetrip is “beating” Airbnb in Europe. More detail in the comments.

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But the service has only continued to grow in popularity, adding two million more members in the last year to get to 5 million members. And now, it’s looking to build upon that momentum with a new $15 million Series B investment, led by General Catalyst Partners with help from Menlo Ventures and existing investors Benchmark Capital and Omidyar Network. The San Francisco-based company will use the new money to shore up its infrastructure, improve its products and hire more employees.

CouchSurfing often gets mentioned along with Airbnb, which also helps people find a place to sleep from fellow users. But CEO Tony Espinoza said the goal of CouchSurfing has been about more than just finding a place for someone to sleep. The service is looking to build a global human travel network and foster connections between users, so hosts aren’t just making their beds available, but welcoming potential friends into their lives. Users have already had more than 10 million face-to-face encounters and many are participating in 40,000 interest groups on CouchSurfing.

The basic service is free, but users can pay a one-time fee of $25 to verify their identity and location, something many serious users pay for. Espinoza said CouchSurfing has been generating significant revenue and was not in danger of exhausting its previous funding. But he said the chance to get backing from General Catalyst, which has invested in Kayak and Airbnb, and Menlo Ventures, whose managing director Shervin Pishervar has been an vocal advocate of collaborative consumption, was too good to pass up.

The new money, however, may raise more questions from die-hard fans worrying about the startup becoming more corporate. With $22.6 million under its belt, it’s a far cry from the plucky communal startup that was founded in 2004 and survived on donations and code contributions from users. Now, it has 30 employees, and it’s churning out improved products including the recent addition of new mobile applications.

Espinoza said even with its growing following, CouchSurfing is still just scratching the surface of what it can become. He said while there’s a need for more transactional services like Kayak and Airbnb, CouchSurfing is serving a big opportunity by bringing people together through travel.

“When people want to have a great time, the best way to do that is through people,” Espinoza said. “With Kayak or Airbnb,  when you meet people through those services it’s not a natural act to add them as a friend on Facebook. When you use CouchSurfing, you use it to find people,” Espinoza said.

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Thanks to the rise of online communities – and the declining economy – it’s never been easier for individuals and businesses trade unwanted and unused goods and spaces for things that they value. (Even cable network A&E seems to have picked up on a trend with its series “Barter Kings,” launched last month.)

“I believe that we are living through a collaborative revolution as significant as the industrial revolution,” said author and collaborative consumption guru Rachel Botsman, at the TEDGlobal conference in Edinburgh last week. “It is taking us back to the old market principles of sharing swapping, bartering.”

Driven by an interest in authenticity and community, she said people are gravitating to services like Airbnb, TaskRabbit and Liquidspace that let people more efficiently exchange goods and services.

To be fair, most collaborative consumption startups are not based on bartering, even if they employ the same principles of trust and reputation. But it seems like a growing number of recently launched companies do indeed rely on old-fashioned in-kind exchange or support a form of it on their platforms. Here are five new startups that are part of the nostalgic trend.

Bondsy
Inspired by founder Diego Zambrano’s own experience of needing to get rid of stuff when he moved from Brazil to the U.S. years ago, Bondsy connects people to share and exchange their things. The mobile app lets people take photos of their stuff and then send them out to social networking friends, who can simply interact with the images Instagram-style or make an offer for an exchange. What’s really interesting is at this point, Zambrano wants to build the community around non-financial transactions. So instead of offering a friend $50 for an old chair, you could offer to take them to dinner. Bondsy was one of the most recent startups to graduate from the NYC TechStars program, and when I met Zambrano, he told me that things should either be beautiful or useful to people. If not, there’s Bondsy. The app won’t be released until later this summer, but people can sign up for the launch here.

CasaHop
Launched last month by former Huffinngton Post CTO Paul Berry, the Airbnb-ish site lets people swap homes and apartments with friends for free. “Airbnb is more about making yourself a mini-landlord and finding a way to monetize your home. CasaHop is much more about communities and clubs and people exchanging and trading,” Berry told my colleague Ryan Kim. People can use the site for free but the company could make money by charging existing clubs and other communities to allow their members to exchange homes among themselves. Many college and university listservs already serve this function for members, so it makes plenty of sense for them to go after that opportunity.

Smallknot
The crowdfunding startup, which was also part of the recent NYC TechStars class, lets people invest in the businesses closest to their homes. But instead of forcing borrowers to repay lenders in cash, they can return the investment in kind, with special goods, perks or private parties (even an oil painting, in one case). The average loan size for Smallknot tends to be under $15,000, but they fund specific projects, such as a new oven for a local restaurant. The startup grew out of Greenville, S.C. but has launched a couple of pilots in Brooklyn, so far.

99dresses
Launched in March, the Y Combinator startup promises an “infinite closet of free clothes.” Through the online platform, users upload pictures of unwanted clothes, which others on the site can buy with virtual currency. If people don’t have enough “buttons,” they can purchase them for $1 but, in theory, someone could trade their way to a whole new closet. New York-based Refashioner employed a similar concept until its relaunch last week. Now, users can also buy and sell items. But the swapping philosophy is still alive and well in that users can use credit from sales to purchase other items on the site.

HipSwap
One of the several startups aiming to topple Craigslist and eBay as a new and improved peer-to-peer marketplace, HipSwap mostly gives users a platform for buying and selling secondhand and vintage goods. But the company, which launched its platform in March, said a large percentage of its users also use the platform to trade specific goods with each other. While the marketplace wasn’t build to be bartering network, HipSwap said it’s an open platform that people can use in the ways that are most convenient and safe for them.

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The survey was conducted by professors from University of Pittsburgh, Graduate School of Business and College of Business Administration, and University of South Carolina’s Moore School of Business. The Professors will be using the data to publish a paper called “When Is Ours Better Than Mine? A Framework for Understanding and Altering Participation in Commercial Sharing Systems.”

Basically the findings mean these web sharing companies shouldn’t be creating marketing around the environmental benefits of their services. That won’t bring in new users or encourage customers to share more or use the shared goods more.

In addition, the survey found that the popularity of the services aren’t a motivating factor either to bring in new users. That’s because customers are worried that if services get too popular then having easy access to goods could become harder. So web sharing companies should take note of that, too: don’t use how popular your service is in your marketing.

Airbnb and Zipcar are currently not highlighting an environmental angle to their services. But Airbnb’s CEO Brian Chesky has long noted that the service is about a changing mindset about access trumping ownership. Reducing ownership, cutting down on wasteful purchases, and using goods more efficiently just has a natural environmental angle. And perhaps that’s the best way to deliver an environmental benefit: have it be an effect of a service, not the aim.

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San Francisco-based Threadflip allows users to upload pictures of clothing, shoes, jewelry and bags to be sold on its community market. Shoppers can browse by category and type from a collection of more than 8,000 items. When an item is sold, Threadflip arranges the transaction and helps sellers deliver their goods to buyers. Since the service started in beta in late December, 30 percent of users are active daily and 10 percent of beta users are buying items, Threadflip said.

Threadflip competes with eBay and other collaborative consumption services like Poshmark, which raised $3.5 million in December, and 99dresses, a new Y Combinator company. Threadflip CEO Manik Singh tells me the company is trying to make its mark by offering a high level of service, including an end-to-end service for sellers. Busy users can just send their items to Threadflip and the service will market the clothes, set a price, take pictures and put the items up for sale. Users who upload their own pictures, get prepaid shipping packages with labels and wrapping material for the items they sell. Threadflip takes 15 percent of the sale from the seller’s side.

“What we’re proud of is the end-to-end user experience, online and offline,” said Singh. “Threadflip is really the easiest way to refresh your wardrobe.”

Singh got the idea for Threadflip after his wife had trouble parting with a pair of expensive boots she no longer wore. Singh, a former VP of product management for Causes, co-founded Threadflip last year with a couple of other former Causes employees.

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Soon there will be, as New York City-based Spinlister is two steps ahead of me. It is looking to create a peer-to-peer marketplace for bikes, connecting those who have an excess supply with people who want to ride but aren’t quite ready to own or buy their own two-wheeled vehicles yet. The service, which is in a private beta now, will officially launch in New York City and San Francisco starting April 1.

Spinlister, of course, is part of a larger trend toward collaborative consumption — like what Airbnb is doing for those who wish to rent rooms in their house or even their entire apartments when out of town, or how RelayRides helps users rent their cars while they’re not using them.

Once the site launches, anyone will be able to make his or her bike available for rent. Using Facebook Connect for authentication, Spinlister will let bike renters and bike owners connect to discuss how long a bike will be rented for, and how the thing will be handed off. Like Airbnb and similar services, it will rely on its community to provide ratings — both of the bikes themselves and those who rent them.

While NYC and SF are its launch cities, Spinlister has quietly received interest from all over: Pre-launch, users have listed bikes in 75 different cities and 23 different countries worldwide. The plan is to have a staggered city-by-city rollout based on demand and inventory.

Meanwhile, in its pre-launch period, Spinlister founder Will Dennis has also made his own bike available — it’s a pretty sweet little fixie — and it’s been rented out for 21 days out of the first month that it’s been available.

Granted, he’s cheating a little bit: the Blue Lady Killer, as he calls his bike, is listed for just $1 a day, which is incredibly low by bike rental standards. Other bike owners have been a bit more conservative with their rates, with the average listed rental price running around $20 a day. That’s a fair return for a vehicle that would otherwise just be sitting in someone’s garage, gathering dust — or rust. But it’s still less than half the average bike rental fee for most suppliers in major metropolitan areas.

While Spinlister is offering up a platform for peer-to-peer bike lending, it’s also partnering with the U.S.’s two largest bike rental companies to ensure that it’ll have plenty of inventory when it does go live.

Spinlister is just two guys now — Dennis and his co-founder Jeff Noh. (Jeff’s bike is listed here.) But they’ve raised a $425,000 seed round and are looking to hire an iOS developer to take Spinlister mobile, as well as a Ruby on Rails developer.

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