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OMG! One of the co-founders of social-media giant Twitter (TWTR - Get Report) is stepping down from the company's board. Twitter disclosed in an SEC filing that Evan Williams will leave the board at month's end.

Williams, 46, co-founded the San Francisco-based company with CEO Jack Dorsey and entrepreneur Christopher Isaac "Biz" Stone 13 years ago. Williams served as Twitter's CEO from July 2009 through March 2010, but is currently chief executive officer of web publisher Medium Corp. and tech-systems innovator The Obvious Corp.

Twitter, which launched in 2006 as an expanded instant messaging service with a 140-character limit, has struggled over the years in terms of both usage and relevance, losing comparable market share to the likes of Facebook Inc. (FB - Get Report) and, for a time, Snap Inc.'s Snapchat (SNAP - Get Report) . However, changes in recent years like upping the character limit and expanding capabilities to include video, live conversations and other functionality have helped TWTE regain presence among users.

President Donald Trump's prolific and attention-grabbing tweets have also helped reinvigorate the app and drawn in more users and followers. Still, Trump's tweets have sparked criticism of the company for allowing the president to make public remarks that critics consider abusive and disparaging, which would normally violate Twitter's terms of service.

In a tweet, Williams thanked both Dorsey and Stone for "starting this crazy company with me -- and continuing to make it better and better."

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

- Ev Williams (@ev) February 22, 2019

Twitter went public on Nov. 6, 2013, priced at $26 but popping to $45.10 a share on its opening trade. But as of Monday morning, the stock was trading at $32.47, up some 2.4% on the day yet down about 28% from 2013's opening tick.