Twitter Inc. Chief Operating Officer Dick Costolo has taken over the reins from co-founder and Chief Executive Evan Williams.
Williams, who ran the privately held San Francisco company for the last two years, said he plans to focus on product strategy.
"Building things is my passion," Williams said in a blog post on the company's website.
The social-networking site, whose value has been estimated by private investors and research analysts at $1 billion to $2.4 billion, has grown rapidly and now has more than 160 million users. The handoff comes as Twitter has been looking for ways to make money.
Costolo, 47, an early investor in Twitter and a former advertising product manager at Google Inc., joined the company a year ago to help forge search partnerships with Google, Yahoo Inc. and Microsoft Corp., as well as spearhead its advertising initiatives. Twitter recently rolled out a major redesign of its website led by Williams to make the site easier for people to use and to make it easier for advertisers to get noticed.
Twitter, which launched four years ago, is now the world's third-largest social-networking site, behind Facebook Inc. and Windows Live Profile, according to ComScore Inc. Twitter had 96 million unique visitors in August, up 76% from a year earlier, ComScore reported.
Twitter users post more than 90 million updates each day. Twitter's workforce has swelled to more than 300 employees and is sure to grow by hundreds more, analysts say.
That will increase the demands on its chief executive, particularly now that generating revenue is Twitter's top challenge. Twitter has taken its time developing its advertising strategy and still has a ways to go, analysts say. Some advertisers, such as Virgin America, have had success advertising on Twitter, but others aren't yet sure.
Costolo is Twitter's third CEO. Williams took over that post in 2008 from co-founder Jack Dorsey, who remains chairman and now runs another company, Square. Costolo joined Google in 2007 when he sold his company FeedBurner — which distributed podcasts, blogs and other Internet content — to the Internet search giant.