Twitter co-founder Evan Williams stepped down Monday as chief executive, handing over to a Google veteran brought in last year to help the micro-blogging service make money.

Williams, in a post on the company blog, said Twitter's chief operating officer, Dick Costolo, would take over as CEO of the San Francisco-based startup effective immediately.

Costolo, whose Web content distribution company Feedburner was purchased by Google in 2007, has been at the forefront of efforts to begin monetizing Twitter since he joined the company last year.

Williams said he planned to focus on product strategy.

"I am most satisfied while pushing product direction," he said. "Building things is my passion, and I've never been more excited or optimistic about what we have to build.

"This is why I have decided to ask our COO, Dick Costolo, to become Twitter's CEO," he said. "Starting today, I'll be completely focused on product strategy."

Williams said Costolo has been a "critical leader in devising and executing our revenue efforts, while simultaneously and effectively making the trains run on time in the office.

"Given Dick's track record as a three-time successful CEO, I'm confident we can make this a smooth transition," he said.

Twitter, which allows users to fire off messages of 140 characters or less known as "tweets," has enjoyed skyrocketing popularity since it was launched in 2006 by Williams, Jack Dorsey and Biz Stone.

Williams said Twitter now employs around 300 people, up from 20 when he took over as CEO from Dorsey two years ago. Dorsey is presently Twitter's chairman while Stone serves as "creative director."

"In those same two years, we grew from three million registered users to more than 160 million today," Williams said. "By all accounts Twitter is on a roll. We've redesigned our website to great user feedback.

"Our user and usage numbers are growing at a rapid clip all around the world," he said. "We've launched an early, but successful, monetization effort."

The Twitter co-founder added that "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," he said.

Williams' departure comes just three weeks after Twitter unveiled an overhauled website that lets people more easily sift through the growing mountain of micro-messages and creates more opportunity for advertising.

Twitter's "Promoted Tweets" advertising service allows companies and others to place "tweets" at the top of a page of search results.

Dan Frommer of Silicon Valley technology blog BusinessInsider.com said Williams had done a good job scaling up the company and it was "time to use Twitter's hit product and huge audience to print cash."

"It's clearly Dick Costolo's job now to turn Twitter into a real business, judged by the amount of revenue it can generate, not just the amount of tweets it can generate," he said. "And it's obvious that he's a better man for that job than Williams is."