Twitter co-founder Evan Williams is throttling back his role in the hot microblogging startup and turning his attention to a new entrepreneurial endeavor.

"After stepping down from CEO six months ago, my mind started to wander," Williams said in a message at his EvHead blog on Wednesday. "I've decided to scale back my role at the company."

The message by Williams came on the heels of Twitter co-founder Jack Dorsey announcing that he was returning to the high-flying company as executive chairman to head its product team.

When he ceded the Twitter helm to then chief operating officer Dick Costolo, Williams said he would be focusing on product strategy at the San Francisco-based firm.

"Now that Twitter is in capable hands that aren't mine, it's time to pick up a whiteboard marker and think fresh," Williams said.

"There are other problems/opportunities in the world that need attention, and there are other individuals I'd love to get the opportunity to work with and learn from," he continued.

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

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.

Williams said he felt that Twitter is "on solid ground and in capable hands."

"I'm still involved, but it's no longer my full-time job," Williams said of his evolving position at Twitter.

"I'm not ready to talk about what I have planned next," he continued. "But, I will venture a prediction about what's next for Twitter: It will be bigger and better."

Williams said he will remain on the Twitter board of directors and meet frequently with people at the firm.

"I'm not disappearing from Twitter," he stressed.

Twitter has been a media darling in recent weeks as it officially marks its fifth birthday and shines as a tool used by people fighting for political reform in the Middle East and elsewhere.

Twitter's third co-founder Biz Stone was being interviewed by bawdy radio talk show personality Howard Stern when he revealed details of Facebook founder Mark Zuckerberg expressing interest in buying Twitter.

Williams and Stone made a road trip from San Francisco to Facebook headquarters in the Silicon Valley city of Palo Alto to meet with Zuckerberg in late 2008.

On the way Williams and Stone pondered a price for Twitter, and settled on a figure of $500 million tossed out by Stone.

Zuckerberg led Williams and Stone into a room and sat in the only single chair, leaving the Twitter co-founders to share a small love seat.

"It was fun, but it was also awkward," Stone said of the chat with Zuckerberg. "I was making all these jokes, and they were just dying."

Zuckerberg asked the pair to propose a sale price for Twitter, and responded "That's a big number" after Williams tossed out the $500 million figure, according to Stone.

Facebook made a cash-and-stock offer for Twitter during a cordial meeting that included lunch, Stone said.

"When talking to him it was weird," Stone said of Zuckerberg.

"He was one guy but it almost seemed he had 12 people in his head," he added during the Stern broadcast. "He had already gone through every possible scenario that we could talk about."

After returning to Twitter offices that afternoon, Williams sent Zuckerberg an email saying the firm wasn't for sale.

"To be perfectly honest, we really had no intention of selling," Stone said. "We just had to go meet the great Mark Zuckerberg."