Twitter has elevated chief operating officer Dick Costolo to CEO, replacing founder Evan Williams, who will now focus on product development.
The move signals that Twitter is set to expand as an independent company, as Costolo has led its development of an advertising platform. The former Google executive joined Twitter as COO a year ago. Since then, Twitter has solved many of its technical issues, developed ad products and released a revamped version of its service.
"When I insisted on bringing Dick into the COO role a year ago, I got a lot of questions from my board," Williams wrote in a blog post. "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."
Costolo recently brought on former Fox executive Adam Bain to lead the company's ad sales efforts.
Twitter now has 300 employees, and Williams reports the service now has 165 million users producing 90 million tweets a day.
Not everything has gone smoothly for Costolo, however. In addition to Promoted Tweets and Promoted Trends, Twitter tried to cash in on the online flash-sales craze with @earlybird. It hasn't gained much traction, and Twitter plans to phase it out less than three months after its launch.
Costolo founded RSS ad network Feedburner, which Google acquired in 2007.
"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," Costolo said in a statement.