31, 36, 35 | COFOUNDERS

The brain trust behind Twitter built their HQ, in San Francisco's SoMa district, to be just like their application: open, minimal, and extremely utilitarian. The microblog site may largely be the domain of tweens and teens now, but the idea for it came to Jack Dorsey when he was writing software for courier and taxi companies in need of real-time status updates. Evan Williams (the business guy) and Biz Stone (the creative one) made the concept a reality. "With Jack's idea and the proliferation of IMing, we felt that this could appeal to groups of friends who wanted to stay in touch," Williams says. Even the founders have been shocked by Twitter's impact on campaigning, relief operations, and news-gathering (as shown during the Mumbai attacks and the miracle landing of US Air 1549). More than 6 million individuals and entities from Demi Moore to GM are now Twittering. "It definitely is bigger than we thought," says Williams, citing a 900 percent increase in users last year. Bigger does not mean a better balance sheet, as many technorati point out, and it's unclear whether the new revenue model—based on charging companies to certify their tweets—will ever justify their having walked away from the $500 million buyout offer from Facebook. But Williams, Stone, and Dorsey have no regrets. "We've scaled the technology mountain," Stone says. "Now we have to focus on the business. We want to remain independent. We want to prove ourselves."

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