San Francisco (CNN Business) Twitter co-founder Evan Williams is leaving the company's board, according to a document filed with the Securities and Exchange Commission on Friday.

Twitter TWTR Williams, who formerly served as CEO offor two years until 2010, will step down at the end of February.

"It's been an incredible 13 years, and I'm proud of what Twitter has accomplished during my time with the company," Williams said in a statement published in the filing. "I will continue rooting for the team as I focus my time on other projects."

On Twitter, Williams tweeted his thanks to his co-founders, Jack Dorsey and Biz Stone.

"Thank you, @jack and @biz for starting this crazy company with me—and continuing to make it better and better," he tweeted. "And to my fellow board members, new and old—some of the most thoughtful people I've ever known."

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