Evan Williams just posted to the Twitter Blog that he is stepping down as Twitter CEO. The new CEO will be Disk Costolo, which was formerly Twitter's COO. Williams explains that instead he would much rather focus on building great products for Twitter and "pushing product direction".

When I insisted on bringing Dick into the COO role a year ago, I got a lot of questions from my board. But I knew Dick would be a strong complement to me, and this has proven to be the case. During his year at Twitter, he has been a critical leader in devising and executing our revenue efforts, while simultaneously and effectively making the trains run on time in the office. Dick can be even more effective at this now because Ali Rowghani, Adam Bain, Mike Abbott, Katie Stanton and Kevin Thau joined our leadership team this year and are having a big impact. Given Dick’s track record as a three-time successful CEO, I’m confident we can make this a smooth transition.

On top of product direction, Williams added that in order to truly achieve success at Twitter as they continue their substantial growth, he wanted to reevaluate everyone's focus including his own in order to work together efficiently and ultimately, successfully.