The company has so far demonstrated that its combination of subscriptions and advertising revenue can work, although whether it can keep engagement up and continue to grow its international base remains to be seen.

“We exited 2011 having successfully revamped our underlying development infrastructure,” said CEO Jeff Weiner in a released statement. ”Based on that investment, we said that 2012 would be a year of accelerated product innovation, and it was. The products we delivered throughout the year drove member engagement and financial results to record levels in the fourth quarter.”

The company has made a strong international push this year, although 62 percent of fourth quarter revenue still comes from the U.S. The company ended the year with 202 million registered users and strong growth on mobile, along with everyone else.

Mike Isaac for AllThingsD wrote about the issues the company will have to overcome: “The biggest question going forward, however, is if LinkedIn can keep those 200 million-plus users on its platform actively engaged on a regular basis. As I argued previously, one of LinkedIn’s major challenges is to convince its users that it’s a daily destination site to return to for business insights, updates and opinion, not just a repository for one’s online resume.”

Tim Bradshaw for the Financial Times wrote that the company’s influencer program could play into future bets: “Changes included a revamp of its profile pages and a new blogging platform called “LinkedIn Influencers,” which is used by prominent members such as President Barack Obama and Sir Richard Branson, the prominent UK businessman, which boosted traffic.”

Bloomberg’s Ari Levy noted that the company’s stock rose as high as 12 percent in late trading on Thursday, and a Bloomberg analyst spoke to the reasons LinkedIn posted such strong earnings: “Investors clearly like LinkedIn’s mix of advertising and fee revenue, both of which are experiencing exceptional growth,” said Paul Sweeney, a Bloomberg Industries analyst.

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Facebook grew to a billion users with a strategy that relied mainly on the desktop web experience for many years. It went public before it even had a decently functioning iOS app. And for a long time it relied on HTML5, which didn’t turn into the winning mobile strategy the company had hoped.

But with investors nipping at its heels, Facebook announced fourth-quarter earnings on Wednesday that finally produced solid numbers when it comes to mobile — a quarter of ad revenue, and, for the first time, proof that daily active users (DAUs) on mobile outnumber those on desktops.

Mobile is crucial to Facebook’s success on almost all levels, but none of it matters unless Facebook is able to capitalize on mobile advertising. As revenue related to gaming and gifts looks uncertain, ads take on a greater importance. Ads made up 84 percent of the company’s revenue this quarter. And if more people check Facebook on their phones than their desktop computers, mobile ads have to play an important part in its strategy. Facebook grew its mobile revenue from 14 percent of all ad revenue in Q3 2012 to 23 percent in Q4 2012, which is a good start; but if those numbers slow, the company could be in trouble.

The huge growth in mobile success in 2012 came not from innovative, brand-new strategies on smartphone platforms, but rather Facebook’s decision to update existing products and improve the user experience. In the Q4 2012 earnings call and at TechCrunch Disrupt in September, Zuckerberg said a faster app alone did wonders from the company’s traffic:

“Often, doing a good job is focusing on basic issues like performance and stability,” he said Wednesday.

But during 2013, Facebook and everyone else will need to think about what exactly makes an app shine on mobile, and then go build it themselves. Eventually, just being faster or bigger might not be enough to maintain growth.

Facebook needs to build the Instagrams, Snapchats, or Vines of tomorrow that actually engage mobile users in a new way. And clearly, Zuckerberg recognizes this:

“Now we’re there. We moved fast and ship new versions of our apps on regular monthly schedules,” he said. “Now the next thing we’re going to do is build really good mobile-first expierences.”

But when he says mobile-first, does he mean an actual mobile device? Nope, don’t hold your breath. There’s no Facebook phone on the horizon, Zuckerberg said.

“The big theme that we’re going to push on is mobile. People keep asking if we’re going to build a phone. And we’re not going to build a phone. Because it doesn’t make sense for us,” he said. “We have a billion people using our product and we have to make Facebook really good across all the devices they’re using.”

Instead, Facebook needs to be the company that solves the mobile ad dilemma, or at least, among the fast followers. The torch has been passed to a new generation of computing at Facebook, and it doesn’t involve the traditional PC.

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