Reproduced with permission from BusinessInsider.com.

Ev Williams is stepping down from his role as Twitter CEO.

COO Dick Costolo be taking over as CEO. Costolo joined Twitter in September 2009.

Williams became CEO when co-founder Jack Dorsey was pushed out of the company.

Williams run at the top of Twitter was mixed. The service exploded in popularity, but it still has a lot of problems.

For instance, Twitter was down for a few minutes right around the time the company announced this news. And we still don’t have access to the new design of Twitter.com, weeks after it debuted.

Costolo has been in charge of Twitter’s money making moves of late. He’s been the public face of the company when talking about promoted tweets and trends. He’s a much better public speaker than Williams, which is good for a CEO.

Before Twitter, Costolo was CEO of FeedBurner, which eventually sold to Google.

Twitter explains the move on its blog:

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 160 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.

And here’s Dick’s perspective:

I love working with Evan Williams. During his tenure as CEO, Twitter has gone from 20 employees to 300, from three million registered users to more than 165 million, and from 1.25 million Tweets per day to 90 million today, and he’s done all this while keeping Twitter’s unique culture intact. I completely respect his decision to focus on product strategy and vision. As we’ve all see with the new Twitter, when Ev focuses on product, something amazing can happen. I’m certainly excited to be taking on this role. You couldn’t ask to take a job like this at a better time – 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.

Reproduced with permission from BusinessInsider.com.