Facebook CTO Bret Taylor: “Mobile is the primary focus for our platform this year.”

If any consensus emerged around the state and health of the Facebook platform, it was that the future of it is clearly on mobile devices.

Facebook chief technology officer Bret Taylor said today that his primary focus in 2011 will be building out the platform’s mobile presence. Top executives from the ‘Big 4’ developers — Zynga, Disney’s Playdom, Electronic Arts’ Playfish and Crowdstar — also said they’re moving their attention to mobile platform this year in a panel earlier today at our Inside Social Apps conference in San Francisco.

While Facebook has permeated half of the top websites in the world and fueled the growth of multi-billion dollar gaming companies like Zynga, its platform lags behind in reaching even the very best-selling mobile applications.

Taylor said the company had tremendous engineering challenges in ensuring an even experience across its all of its web and mobile presences. He said every time the company rolls out new features, it has to add them to seven versions of the same product: Facebook.com, m.facebook.com (which is optimized for low-end phones), touch.facebook.com (for higher-end, touch-enabled phones), the iPhone app, the Android app, Blackberry and numerous custom integrations of Facebook on other mobile devices.

“It’s incredibly challenging,” he said. “You end up picking and choosing platforms even though your goal is to reach everyone.”

At the same time, phones are inherently social. They’re single-user devices that come filled with a person’s contacts and friends. Plus, their primary function is as a communications device, he said. On top of that, they have access to location.

So far, Facebook has been gradually building out a set of tools that mobile developers can use to populate their applications with people and their friends. Taylor said Flixster’s integration of single sign-on, a quick way for users to sign-up for apps and import their friends, drove a 300 percent increase in sign-ups.

One way Facebook will be tackling the problem is HTML5, which will make it easier for engineers to quickly iterate and release features to a broad set of users without having to go through an approval process.

“We’re still maybe a little ahead of that curve, but we’re making a huge amount of investment,” he said. “Most people in Silicon Valley view HTML5 as the future.”

At an earlier panel, executives from the very biggest social gaming companies also said they’re starting to invest more in mobile platforms. Crowdstar chief executive Peter Relan said one-third of the company’s production will be focused on mobile.

Playdom co-founder and Wild Needle chief executive Rick Thompson said that if he were a smaller, less capitalized developer today he would opt for mobile first.

“Long-term, I couldn’t be more bullish on Facebook,” he said. “But if you have lower budget and quicker need for success, go mobile.”

Kristian Segerstrale, the chief executive and co-founder of Playfish, said that Facebook’s platform had much to improve on with its capabilities for mobile developers.

“The Facebook mobile experience remains high-friction in terms of actual DAUs,” he said. “I think they have a place. But Facebook, the handset providers and service providers will get better at merging. I think consumers will choose the best social experience — and it would be surprising if it didn’t turn out like social on Facebook.”