“During his year at Twitter, [Costolo] 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,” Williams said.

Costolo told the Wall Street Journal the shift occurred because “it became clear to everybody that Ev’s got to be able to focus on product strategy.”

Williams co-founded the service in 2006 with Jack Dorsey, who now runs the mobile payment platform Square.