NEW YORK (CNNMoney.com) -- Twitter co-founder Evan Williams is stepping down as the company's CEO, with COO Dick Costolo taking over as the company's new chief executive.
Promoting Costolo frees Williams up to be "completely focused on product strategy," Twitter's creator wrote in a blog post announcing Costolo's new role.
"Building things is my passion, and I've never been more excited or optimistic about what we have to build," Williams wrote.
Costolo says he and Williams are "perfectly complementary" business partners.
"He takes a very long view and thinks very hard about the product strategy and the product vision," Costolo told CNNMoney.com in a recent interview. "I kind of take the 'here are the things we need to do day-to-day operationally, and this is what we need to have done by tomorrow and this is the way we need to measure that to move forward.' So the two of us working together works beautifully."
Over the last couple months, Twitter has rolled a number of major changes to its four-year-old messaging system. On Monday, just a few hours before naming its new CEO, Twitter launched "promoted" accounts, using an algorithm to link advertisers with users likely to be receptive to their message.
In September, Twitter completely revamped its site to allow users the ability to embed photos and video clips into their tweets. The "new" Twitter, which is gradually rolling out to users, also includes a sidebar that shows users more information when they click on a tweet.
As the company has expanded, so has its business model.
"I think that the advertising platform will be one pillar of the monetization strategy, and then we'll have a commercial pillar," Costolo said recently. "The commercial pillar will involve things like ... making it easier for people to buy things on Twitter, making it easier for people to do things like make reservations, select coupons for local offers -- that sort of thing."
Costolo -- who already has a hash tag, #newtwitterceo, to go with his new title -- says it's the ideal time for him to step into a new role at Twitter. The company now has 300 employees supporting 165 million registered users, firing off 90 million tweets a day.
"The team is incredible, we have awesome stuff in the pipeline, and we're ready to accomplish more in the next two years than we've accomplished in the last four," Costolo said in a written statement.
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