• by Gavin O'Malley  @mp_gavin, October 4, 2010

Twitter on Monday tapped Chief Operating Officer Dick Costolo to run the company as CEO. The decision was made by Twitter's existing CEO Evan Williams, who has decided to focus his efforts entirely on "product strategy."

"I am most satisfied while pushing product direction," Williams said in a blog post on Monday -- a realization that Williams said he had during Twitter's most recent upgrade. "Building things is my passion, and I've never been more excited or optimistic about what we have to build."

Since Williams took Twitter's reins two years ago, the micro-blogging service has grown from roughly 20 to 300 employees and 3 million registered users to over 160 million, and has seen daily tweets increase from about 1.25 million to 90 million. Yet the company's monetization strategy remains a work in progress. Williams himself described the recently revamped effort as "early, but successful."

Since debuting its highly anticipated ad products in April, Twitter has signed over 30 top brands, including Coca-Cola, Virgin America and Starbucks. Yet "some marketers say that early results are promising but that advertising on Twitter remains an experiment," The Wall Street Journal reported last week. "Other marketers, including PepsiCo's beverage brands and Best Buy, who tested out Twitter's new ad products haven't made new ad buys."

"The challenges of growing an organization so quickly are numerous," Williams said on Monday. "Growing big is not success, in itself. 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."

Since joining Twitter a year ago, Costolo is credited with developing and executing revenue efforts, while -- as Williams explains -- "making the trains run on time in the office."

Going forward, "we have awesome stuff in the pipeline, and we're ready to accomplish more in the next two years than we've accomplished in the last four," Costolo said in a statement released Monday. For one, Twitter plans to launch a self-service ad platform next year, Costolo said during the IAB Mixx conference last week. During the conference, Costolo explained marketers' need for additional ad outlets, and the increase in adoption rates for services already offered by Twitter.

In his new role, Costolo will be supported by recent hires, including Ali Rowghani as chief revenue officer in February; Adam Bain as president of revenue in April; Mike Abbott as VP of engineering in April; and Katie Stanton as VP of International.