Evan Williams steps down as Twitter CEO, COO Costolo gets nod

Responding to the increasing size of Twitter, and a desire to go back to his roots in development, Evan Williams said Monday that he would be stepping down as Twitter's CEO, handing the company over to current COO Dick Costolo. Williams had served in the position since 2008.

The change would take effect immediately, Williams wrote in a blog post Monday. "Starting today, I'll be completely focused on product strategy," he said. "Building things is my passion, and I've never been more excited or optimistic about what we have to build."

Costolo founded Feedburner, which was acquired by Google in 2007. He spent some time at Google as product manager for its lucrative advertising business, but joined Twitter last year after Williams lobbied the board to bring him in.

"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," he said. "Given Dick's track record as a three-time successful CEO, I'm confident we can make this a smooth transition."

Williams stepping down may also make good business sense: he started as CEO with a team of only 20 employees.

Since then, Twitter has grown to more than 300, so bringing on a chief executive with more leadership business as well as experience in advertising -- something the company must look into in order to generate more revenue -- seems to be a good fit. Costolo has said monetization is key for the future of the company.

"I think that the advertising platform will be one pillar of the monetization strategy, and then we'll have a commercial pillar," he recently told CNNMoney.com.