James Martin

Bret Taylor, who has been Facebook's very public CTO for three years, today announced that he's leaving the social-networking giant to start a company. He didn't say what his next venture will be.

This is a big loss for Facebook and comes less than a month after the company went public. While Facebook stock rose more than 6 percent today, to close at $30.01, it's still 21 percent below its offering price of $38 a share. The news about Taylor, first reported by AllThingsD, came out after the market closed.

Naturally, Taylor turned to Facebook to tell the world of his career change. He wrote:

I wanted to let you all know that I'll be leaving Facebook later this summer. I'm sad to be leaving, but I'm excited to be starting a company with my friend Kevin Gibbs. While a transition like this is never easy, I'm extremely confident in the teams and leadership we have in place. I'm very proud of our recent accomplishments in our platform and mobile products, from Open Graph and App Center to Facebook Camera and our iOS integration. I'm even more excited for the world to see all the amazing things these teams have coming.

Taylor gives special thanks to CEO Mark Zuckerberg, who, in a statement, said, "I've really enjoyed working with Bret and getting to know him as a friend and teammate. I'm grateful for all he has done for Facebook and I'm proud of what he and his teams have built. I'm also proud that we have a culture where great entrepreneurs like Bret join us and have such a big impact."

Behind the hugs and public thanks, however, there has to be some anxiety at the company and potentially among investors. Taylor has been an important leader at Facebook, in charge of both platform and mobile, and Facebook is under intense pressure to figure out ways to make money from its growing mobile user base.

Taylor came to Facebook though an acquisition. He was the co-founder and CEO of FriendFeed, which aggregated social network feeds. Facebook bought the company in 2009 for about $50 million. FriendFeed grew out of work he did while an Entrepreneur in Residence at Benchmark Capital. Before joining Benchmark, Taylor was a product manager at Google, where he co-created Google Maps and Google Local.

Updated at 2:37 p.m. PT with more background and analysis.