Twitter co-founder Ev Williams has stepped down as CEO of the social networking phenomenon to concentrate on product strategy. COO Dick Costolo will replace Williams.

Costolo has only been with the firm for about a year and was initially hired to help devise and execute a revenue model. Although Twitter is currently the hottest social network around the company is aware that it has to turn that popularity into profit.

"The challenges of growing an organization so quickly are numerous. Growing big is not success, in itself," Williams wrote on the company’s blog. "Success to us means meeting our potential as a profitable company that can retain its culture and user focus while having a positive impact on the world."

Claiming that he is, "most satisfied while pushing product direction," Williams said that Costolo is the right man to drive revenue at Twitter. "When I insisted on bringing Dick into the COO role a year ago, I got a lot of questions from my board. But I knew Dick would be a strong complement to me, and this has proven to be the case. During his year at Twitter, he has been a critical leader in devising and executing our revenue efforts."

The new business model includes deals with Google, Microsoft Bing and Yahoo to show tweets in search results, and promoted tweets, which enable companies to pay to appear in related search results.

Costolo founded Feedburner, which was acquired by Google in 2007. He became an investor in Twitter in 2007, which gained him a place on its board.

Williams founded Twitter with Jack Dorsey and Biz Stone in 2006. Since then it has grown to over 165 million registered users posting 90 million tweets per day. It now employs around 300 people. Williams took over the CEO role from Dorsey in 2008.