Here’s one that we’re catching up on after a wild Oscar Friday: Evan Williams, one of the four co-founders of Twitter, is exiting as a member of its board of directors, ending his tenure at the company that helped fuel the explosion of social media.

Twitter confirmed the news Friday in an SEC filing, saying Williams would leave at the end of February and that it is “grateful to Mr. Williams for his service.”

“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 the filing. “I will continue rooting for the team as I focus my time on other projects.”

Williams is currently CEO of Medium, a web-publishing platform, and also founded tech innovator Obvious Corp that includes a venture arm Obvious Ventures.

Williams launched Twitter in March 2006 in San Francisco with now-CEO Jack Dorsey, Biz Stone and Noah Glass. Williams and Glass had hired Dorsey for their podcast company Odeo and launched the 140-character-per-post platform, Dorsey’s new idea, with leftover investor cash. Dorsey and Stone now remain from the original quartet.

In its fourth-quarter earnings report earlier this month, Twitter said its user count is 321 million worldwide.

“I appreciate you, Ev! You’re the reason I joined Odeo in the first place,” Dorsey tweeted Friday. “I’ve learned so much from you since that crazy interview you and @Noah put me through. We’re going to miss your voice in our board conversations. ❤️”

On Friday Williams expanded on his time after the news was official: