Mayer confirmed the acquisition with an Instagram picture (no Flickr love?) with members of the Stamped team. The deal would reunite Mayer with Stamped CEO Robby Stein, who used to work for Mayer at Google as a product manager.

Mayer has talked about becoming more of a mobile company but also said the Yahoo would be making smaller acquisitions. This deal, the details of which we’re still trying to nail down, would satisfy both goals unlike a big acquisition like Foursquare. Stamped’s team will now form the basis of a Yahoo mobile product and engineering office based in New York.

Stamped began life as a recommendation app that allowed people to stamp the things they like, such as restaurants, movies, TV shows and books. It took on $3 million in funding including $1.5 million a few months ago from a bunch of celebrities including Justin Bieber, Ryan Seacrest and Ellen Degeneres.

Stamped released a statement explaining that Stamped users will soon get a tool to export their stamps. In the future, Stamped will be working on more mobile tools for Yahoo and will be hiring a lot of engineers.

“As for our team’s next step, we couldn’t be more thrilled to join Yahoo!. As a team of mostly former Googlers, we’ve all worked with and are big fans of Marissa. So when an opportunity arose to become a part of the team at Yahoo!, we jumped. As entrepreneurs, it’s never easy to walk away from something you built from the ground up, but the folks we met with at Yahoo! are simply top-notch and we’re thrilled to be joining them!”

The buy is another sign of Silicon Valley companies buying up New York talent and extending their reach into the city. Square recently bought design firm 80/20 and eBay bought Hunch.

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Social networks will displace business processes, not socialize them
• Social first-quarter 2013: analysis and outlook
• Siri: Say hello to the coming “invisible interface”

But the reality is, most of us usually look to our friends and family for much simpler input on things that they’ve bought or experienced. Was it good, or not? Was it worth it? That is exactly what a new iPhone app called Stamped aims to help people do.

User reviews that are simple and social

The way Stamped works is simple: You use the app to give a “stamp” of approval to something you’ve bought or experienced and liked, such as a book, movie or restaurant. Each stamp can be commented on by other app users. Stamped makes money by taking a cut each time a user buys something on sites such as OpenTable, Amazon, iTunes (aapl) and Fandango based on another user’s recommendation.

Stamped screenshot (click to enlarge)

The app features a constantly updating list of stamps from your social contacts, and it aims to maintain quality and combat spam by only giving new users 100 stamps each. Users earn the right to more stamps based on the quality of their recommendations. It is currently available only on iPhone, but Stamped says Android, iPad and web versions are in the works right now.

Stamped co-founder and CEO Robby Stein says the app wants to keep it simple by only having people recommend products and experiences that they wholly endorse — not things that are mediocre or bad. Stein said in an interview Monday:

“We’re trying to very much just replicate what happens when you talk to someone in real life and a great thing that you recently did comes up in conversation. For the most part, you don’t recommend bad things to people. We’re trying to take a very social, natural approach to this, where every stamp starts a conversation.”

But which recommendation app will win?

To me, what Stamped does makes a lot of sense: I hate sorting through a bunch of long reviews to try and suss out whether I would like something or not. What does three out of five stars really mean when coming from a random Yelp reviewer anyway? But Stamped is not the only app gunning to take down the flawed but dominant five-star rating system for online reviews. For example, Wikets, an iPhone app that launched earlier this month, does a strikingly similar thing; Kevin Rose’s new startup Oink does as well. Foursquare’s “list” feature that launched earlier this year allows people to give their approval to great restaurants and shops in a simple way. An iPad app called Ness debuted a few months ago with a similar value proposition of tailored recommendations based on social cues. That’s just naming a few of the players in this new wave of super simple, super social review apps.

Of course, with startups the execution is just as important as the idea, and Stamped stands a great chance of succeeding based on the pedigrees of its founders, investors and advisors. Robby Stein and his co-founder Bart Stein are both ex-Googlers, and Stamped’s third co-founder is Kevin Palms, who previously led development of risk analytics technology at a New York City hedge fund. The company is backed with money from Google Ventures and Bain Capital, and counts Instagram’s Kevin Systrom as an advisor and celebrity chef Mario Batali as an early user and advisor.

One thing is for sure: The era of the five-star rating system for user generated review appears to be coming to an end. But which company will lead the review system of the future is still up for grabs.

Thumbs up photo used courtesy of Flickr user Sarah Reido, or “.reid.”

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Facebook’s IPO filing: ideas and implications
• Frenemy mine: The pros and cons of social partnerships for online media companies
• The evolution of consumer-media cloud storage