Costolo to be new Twitter-in-chief

Twitter co-founder Evan Williams is to step down as CEO, while COO Dick Costolo will take over as the company's new chief executive.

Costolo's assignment before Twitter was as founder and CEO of media distribution service Feedburner, which was acquired by Google.

Costolo's promotion allows Williams to be "completely focused on product strategy," according to a blog post announcing Costolo's new role, by Twitter's creator.

"Building things is my passion, and I've never been more excited or optimistic about what we have to build," Williams wrote.

According to Costolo he and Williams were ''perferctly complementary'' business partners.

Costolo told CNNMoney.com in a recent interview that Williams took a very long view and thought very hard about the product strategy and the product vision. He added that he, (Williams) took more of the operational day-to-day things approach and what needed to be done by the next day and their working together therefore worked beautifully.