Micro-blogging site Twitter today named Dick Costolo as its new Chief Executive Officer after incumbent CEO Evan Williams stepped down.

Costolo, who was Twitter's Chief Operating Officer and one of its early investors, joined the company last September and has been leading its advertising initiatives.

Williams, who co-founded Twitter, will be "completely focused on product strategy."

"I am most satisfied while pushing product direction.

Building things is my passion, and I've never been more excited or optimistic about what we have to build," Williams said in a blog post on Twitter.

Williams, 38, said while the company has grown rapidly, "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. This is no small task."

Twitter currently has 300 employees, up from 20 when Williams took over as CEO two years ago.

Private investors have valued the company at more than USD 1 billion dollars.

Its registered users currently stand at more than 165 million today, compared to three million two years ago. In addition, the number of Tweets -- the 140 character messages people post on Twitter -- has grown to 90 million a day from 1.25 million tweets two years ago.

Twitter.com had 96 million unique visitors in August, up 76 per cent from a year earlier, according to research firm ComScore Inc.

"By all accounts, Twitter is on a roll. Our user and usage numbers are growing at a rapid clip all around the world. We've launched an early, but successful, monetisation effort," Williams said.

During his year at Twitter, Costolo has been a "critical leader" in devising and executing its revenue efforts.

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

Costolo, 47, is a former management consultant and ad specialist.

He was formerly an advertising product manager at Google Inc.

As part of its efforts to woo marketers, Twitter has hired advertising executives from Google and Facebook, among other companies.

Since launching its advertising offerings in April, Twitter has signed on more than 30 big-name brands, including Coca-Cola and Verizon Wireless. .

witter had last month unveiled a revamped website that made it easier for users to check images and videos.

The slew of new features were aimed at attracting more advertisers to Twitter.com, which faces stiff competition from social networking site Facebook.

The company has been developing a platform for advertisers that want to promote products and services.

Twitter was launched four years ago. Williams took over as CEO from co-founder Jack Dorsey after the company's board felt he could do a better job expanding the company at that stage.

The majority of its users are from outside the US, in countries such as Japan, South Korea, Brazil and Indonesia.