Evan Williams, co-founder of Twitter, will be leaving his position as the CEO of the company. He is turning the reins over to COO Dick Costolo. Williams says the move will allow him to focus on product strategy and his passion of building things.

Costolo came to Twitter a year ago when his company, Feedburner, was acquired by Google. He has stated that he and Williams are a compliment to each other when working together, and that they make a great team.

Twitter has been steadily expanding its services and adding new features over the last several months. September saw the addition of embedded photos and video clips being added to tweets. They are also rolling out a new sidebar that shows more information to users.

A new feature launched on Monday, “Promoted Accounts”, is designed to allow advertisers to be linked with users that will likely need their products. The advertising arena is one that Costolo has said he hopes will revamp Twitter. He plans to make it easier for users to purchase, reserve, and snag, merchandise through the service.

Twitter currently has 165 million users that tweet a total of 90 million times a day.