Williams will not be leaving the company, however. He indicated that he will focus on product strategy.
“I am most satisfied while pushing product direction,” Williams said, in a statement. “Building things is my passion, and I’ve never been more excited or optimistic about what we have to build.”
Costolo, founder of RSS feed and management company Feedburner, which was purchased by Google in 2007, was brought aboard Twitter last year by Williams to find ways to monetize its business model. Costolo was a key player in developing Twitter’s “promoted tweets” advertising model introduced earlier this year.