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Mark Zuckerberg's loved ones want you to know he's not evil. Growing up in suburban Dobbs Ferry, N.Y., he was a whiz kid with a heart. When his dad-who ran his dentistry practice on the first floor of the family home-was being run ragged heading up and down stairs, 11-year-old Mark "built an intranet called ZuckNet," says sister Randi, 28. "Anyone could log into any computer around the house and send a message." By then it was clear that her baby brother, who was beating her at the video game Mario Kart by thinking "10 levels ahead," was going places: "It was the first example of when he started building things-and he never stopped."

No kidding. At just 26, Zuckerberg is the tycoon behind Facebook, the social networking website he founded as a Harvard undergrad that's now worth more than $20 billion with 500 million users-1 out of every 13 people on Earth. But being a mogul has a downside. This month The Social Network, the movie version of Zuckerberg's story, hit theaters-and it's hardly a flattering portrayal. As played by Jesse Eisenberg, Zuckerberg creates Facebook to win girls and popularity. Says Eisenberg: "The character is a guy who is desperately trying to fit in."

Zuckerberg has called the film "fiction." Loved ones say he's nothing like the jerk onscreen. He lives a frugal life with college girlfriend Priscilla Chan in Palo Alto, Calif., where they rent a sparse house, shop at Target, and he drives an Acura. Aside from Mandarin lessons and a personal trainer, "Mark is not somebody who has a lot of interest in spending money-to put it mildly," says friend Matt Cohler.

One of four children of Karen, a psychiatrist, and Edward, who still pulls teeth in Dobbs Ferry, Zuckerberg recalls, "My parents worked really hard to make sure me and my sisters could go to good schools." At the elite Phillips Exeter Academy, Zuckerberg was socially "awkward," says a classmate. And a workaholic. On Saturday nights, "he'd be hammering away at the computer or with a math book."

In his sophomore year at Harvard, Zuckerberg launched Facebook. Then things got sticky: In 2004 a trio of former classmates sued him for co-opting their idea. (They settled out of court.) Tyler Winklevoss and his twin brother, Cameron, now 29, were among the three. "The idea that Mark was this genius among geniuses is not true," says Tyler, who shared an early version of a social networking site with Zuckerberg. Still, pals say Zuckerberg was far from Social's calculating drone. Mark "is a lot more emotional than he's portrayed," says Randi. Adds dormmate Rebecca Davis O'Brien: "What's missing from the movie is his fun side."

The other thing friends say is missing from the movie? His heart. Zuckerberg just made a $100 million donation to the struggling public schools of Newark, N.J. Critics slammed the donation as a PR move to counter the film. But Zuckerberg says he just wants other kids to have the same shot at mega-success. How does he see his legacy? He shrugs: "Ask me again in like 20 or 30 years."

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  • With reporting by Kathy Ehrich Dowd/Newark.