Shift at the top of Twitter as a founder steps aside

Evan Williams, the co-founder and chief executive of Twitter, is stepping down to lead product strategy at the company, Twitter announced on Monday. Dick Costolo, the chief operating officer, will succeed Williams.

The company, which announced the change in a blog post, is navigating a period of rapid growth and beginning to build an advertising business.

In the two years that Williams has been chief executive, Twitter has grown to 300 employees from 20; the service's registered users have ballooned to 160 million from three million; and the company has started to generate revenue.

The news was unexpected. Though technology blogs had raised the question of whether Costolo would eventually become chief executive since he was hired a year ago, Williams had always insisted that he wanted the job. But on Monday, he and Costolo both said the decision was Williams's choice and that he wanted to get back to his roots, working on the product. The moves were effective immediately.

Williams said in an interview that he realized he wanted to focus on product strategy while he was working on Twitter's new Web site, introduced during the last month. "I was reminded that maybe that's where my passion and skills lie," he said, "and I couldn't focus on that as much as I'd like."

Fred Wilson, a partner at Union Square Ventures who is on Twitter's board, said, "It's a very smart and gutsy move for Ev to do so." He added that Costolo was "more of a manager, more of an executive, more of a businessman" than Williams.

Though the titles are new, Williams and Costolo had already divided many of their responsibilities along those lines. Costolo has been focusing on business development, like deals with Google and Microsoft, and revenue, including overseeing the introduction of Promoted Tweets and other forms of Twitter ads.

Twitter's new Web site was Williams's brainchild, and he spent much of the last few months working with Twitter engineers on the fine details of building it.

Costolo is a three-time chief executive, most recently at FeedBurner, which he helped to found and sold to Google. He will take over the day-to-day duties of running and expanding the company and continue to be the public face of its financial side. He made the company presentation to advertising executives at the Interactive Advertising Bureau's conference in New York last week, alongside executives from Facebook and Google.

Williams's new role will be a homecoming. He was chief product officer of Twitter before replacing Jack Dorsey, another Twitter founder, as chief executive at the end of 2008. Twitter was founded in 2006 by Williams, Dorsey and Biz Stone, and started as a project inside Odeo, Williams's last company.

Williams and Costolo have been friends for years, and Costolo was an early investor in Twitter. When he moved to the Bay Area last year, he wrote a Twitter post explaining to his mortgage broker that he was unemployed. Williams replied with a private Twitter post asking Costolo if he wanted to be interim chief executive while Williams went on paternity leave.

"I don't know if you're joking," Williams recalls the response from Costolo, who was once an improvisational comedian. "If so, good one. If not, let's talk."