Evan Williams, who co-founded Twitter with Jack Dorsey in 2006, announced unexpectedly in a Twitter blog post that he was stepping down as Twitter CEO to concentrate more of his efforts on “product strategy.” Williams named Twitter COO Dick Costolo as his successor.

In the post, Williams cited Twitter’s rapid growth and development, which include New Twitter, arguably the biggest redesign Twitter has seen since its launch four years ago, in his reasons for the transition. From the blog post:

“By all accounts Twitter is on a roll. We’ve redesigned our web site to great user feedback. Our user and usage numbers are growing at a rapid clip all around the world. We’ve launched an early, but successful, monetization effort. And, many top engineers, product designers, sales people and other key folks have joined our quickly growing team.

In fact, there are 300 people working at Twitter today—compared to about 20 when I took the CEO job two years ago. Back then, people were creating about 1.25 million tweets a day—compared to 90 million today. In those same two years, we grew from 3 million registered users to more than 165 million today.

The challenges of growing an organization so quickly are numerous. Growing big is not success, in itself. Success to us means meeting our potential as a profitable company that can retain its culture and user focus while having a positive impact on the world. This is no small task. I frequently reflect on the type of focus that is required from everyone at Twitter to get us there.

This led to a realization as we launched the new Twitter. I am most satisfied while pushing product direction. Building things is my passion, and I’ve never been more excited or optimistic about what we have to build.

This is why I have decided to ask our COO, Dick Costolo, to become Twitter’s CEO. Starting today, I’ll be completely focused on product strategy.

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.

I’m extremely proud of how far Twitter has come in the last two years. And, I couldn’t be more excited about where our amazing team will take it next.”

Williams’ successor Dick Costolo co-founded and served as CEO for Feedburner, an RSS feed management program acquired by Google in 2007, before joining the Twitter team in 2009. Designated on Twitter by the hash tag #newtwitterceo, Costolo says it’s a great time for him to step into a new role at Twitter.

“The team is incredible, we have awesome stuff in the pipeline, and we’re ready to accomplish more in the next two years than we’ve accomplished in the last four,” Costolo said in a written statement.

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