Twitter Inc., the Internet service for short messages, is developing a penchant for keeping its CEOs' tenures relatively brief, too.

Evan Williams on Monday became the second Twitter co-founder to step aside after a two-year stint, making way for a colleague considered to be better suited for steering the next phase of the company's rapid growth.

Known simply as (at)Ev on Twitter, Williams intends to focus on product development while continuing to work for the company.

The change in command at the privately held company turns over Twitter's leadership to Dick Costolo, an early investor who was hired as chief operating officer a year ago.

Since Costolo's arrival, Twitter has moved into larger offices in San Francisco to accommodate a payroll that has swelled from a few dozen people to about 300 employees.

Costolo, 47, also has played a key role in devising an advertising for Twitter as it tries to make money from an online audience that ballooned from 3 million registered users when Williams became CEO in 2008 to 160 million now.

A recent redesign of the Twitter website opened up more opportunities to sell ads without disrupting the stream of messages by creating a new pane for photos, video and other information, including marketing messages. Williams oversaw the website's new look.

In a joint interview Monday, Williams and Costolo said they believed Twitter had reached another critical juncture that called for different leadership skills.

"I have always been more of a product guy and we are entering a new phase of the company where we require a CEO who is more into operations, sales, finance and operations," Williams said. "And Dick is better at those things than I am."

Williams, 38, took on the CEO job in 2008 because the company's board felt he could do a better job expanding Twitter at that stage than another co-founder, Jack Dorsey. It was Dorsey, still Twitter's chairman, who came up with the idea of broadcasting messages limited to 140 characters and who posted the first message, or "tweet," on Twitter in March 2006.

Costolo's promotion will likely spur speculation about a possible initial public offering of stock from Twitter.

But Costolo, whose Twitter handle is (at)dickc, said in Monday's interview that an "IPO really is not anything we talk about. We have no timetable for it."

Twitter's new leader is much more boisterous than his soft-spoken predecessor. Even so, the two share some common bonds.

Williams, a Nebraska native, can almost come across as shy. That's not a trait associated with Costolo, a former improvisational comedian in Chicago.

Both Williams and Costolo have previously cashed in as entrepreneurs who sold startups to Google Inc.

Williams sold a blogging service, Blogger, to the Internet search leader in 2003; Costolo sold an online advertising tool called FeedBurner to Google in 2007.

It didn't take Williams or Costolo long to realize they prefer running their own companies, so they left Google.

Google is now widely seen as a potential suitor for Twitter. Williams, Costolo and Twitter's third co-founder, Biz Stone, have all said they are interested in maintaining Twitter's independence, although they concede Google or another bidder could make an offer their investors couldn't refuse.

Besides Costolo and its co-founders, Twitter is owned by venture capital funds that have poured about $155 million into the company so far.