Twitter co-founder Evan Williams is stepping aside as the short-messaging service’s CEO. He will turn the job over to Dick Costolo, who has been steering Twitter’s operations for the past year.

The privately held company announced the change in command Monday.

Costolo, an early investor in Twitter, joined the company as chief operating officer a year ago to help manage Twitter’s rapid growth.

Since then, Twitter has moved into a larger headquarters in San Francisco as its payroll swelled from a few dozen workers to 300 employees.

Twitter launched in 2006. Williams became CEO in 2008, after Twitter’s board decided he was better equipped for job than another co-founder, Jack Dorsey.

Williams will remain at Twitter and focus on developing new products.

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