Twitter co-founder Jack Dorsey announced on Monday that he was returning to the high-flying company as executive chairman to head its product team.

"Today I'm thrilled to get back to work at @Twitter leading product as Executive Chairman," Dorsey said in a message on his Twitter feed @jack.

Twitter's chief executive Dick Costolo said in a tweet of his own that he was "excited that Twitter Chairman @jack will be returning to the company day-to-day leading product as Executive Chairman."

Dorsey, who founded Twitter in March 2006 with Biz Stone and Evan Williams, resigned as Twitter's chief executive in 2008.

He remained chairman of the San Francisco-based microblogging service but did not take an active role in daily operations.

In his Twitter message, Dorsey said he would remain chief executive of Square, a company he launched last year that accepts credit card payments on a mobile device.

Twitter spokeswoman Jodi Olson said in an email reply to an AFP inquiry that Dorsey will be assuming "an everyday role to lead our product development."

"The timing is fortuitous," she said. "Not only is Twitter experiencing record growth, but we also now have a new infrastructure that will keep us ahead of that growth and enable us to launch products that will make Twitter more instant, simple and always present."

More than 200 million people use Twitter, firing off more than 140 million messages of 140 characters or less daily. The length limit was set to fit the maximum allowed in text messages sent using mobile phones.

Twitter has been adding employees rapidly as it evolves from a hip startup to a money-making platform for Internet Age communications using smartphones and other gadgets.

Williams in October stepped down as chief executive, ceding the Twitter helm to Google veteran Costolo, who was brought in a year earlier as chief operating officer.

Costolo, whose Web content distribution company Feedburner was purchased by Google in 2007, has been at the forefront of efforts to begin monetizing Twitter. Williams said at the time his focus would be on product strategy.

"Twitter's three co-founders -- Ev Williams, Biz Stone and Jack -- have unselfishly played whatever role was most needed at the time to nurture the company and help the product reach its full potential," Olson said.

"Jack has been involved with the company from day one in various roles," she continued.

Twitter has enjoyed skyrocketing popularity since it was launched in 2006 and now claims to have more than 450 employees, with new hires arriving weekly.

"As executive chairman, Jack will dive in to work with more than 450 people, led by an experienced executive team," Olson said.

It was Dorsey who proposed the idea for Twitter while working with Williams and Stone at podcasting company Odeo.