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Three and a half years ago, Mark Zuckerberg (then a ­Harvard sophomore) and a couple of friends built a ­website to let them share photos and personal profiles with other Harvardians. ­Zuckerberg became CEO of the new enterprise, called Facebook. The social-networking site gradually opened its doors to students at other colleges and then high schools. Now that anyone with an e-mail address can register, the site has more than 30 million members, who use it to blog, share pictures, connect with old friends, and expand their networks.

In May, the company announced the Facebook ­Platform, which lets users build and share tools for personalizing their profile pages and adding, say, videos or music from other websites. The idea, says Zuckerberg, is that the personal connections people have made within Facebook will lead them to content that's interesting to them.

The company is embroiled in a long-standing legal dispute with ConnectU, another networking site that originated at Harvard, over ownership of the initial source code and even the basic business idea. Still, Bloomberg reported in December that the privately held company's value may be more than $1 billion, thanks largely to the site's appeal to advertisers. As Facebook grows, so does its potential to become a major content distributor. Not a bad prospect for a programming project hatched in a dorm room.

--Kate Greene