The C.E.O. of Facebook wants to create, and dominate, a new kind of Internet. Photograph by Carlos Serrao

Mark Zuckerberg founded Facebook in his college dorm room six years ago. Five hundred million people have joined since, and eight hundred and seventy-nine of them are his friends. The site is a directory of the world’s people, and a place for private citizens to create public identities. You sign up and start posting information about yourself: photographs, employment history, why you are peeved right now with the gummy-bear selection at Rite Aid or bullish about prospects for peace in the Middle East. Some of the information can be seen only by your friends; some is available to friends of friends; some is available to anyone. Facebook’s privacy policies are confusing to many people, and the company has changed them frequently, almost always allowing more information to be exposed in more ways.

According to his Facebook profile, Zuckerberg has three sisters (Randi, Donna, and Arielle), all of whom he’s friends with. He’s friends with his parents, Karen and Edward Zuckerberg. He graduated from Phillips Exeter Academy and attended Harvard University. He’s a fan of the comedian Andy Samberg and counts among his favorite musicians Green Day, Jay-Z, Taylor Swift, and Shakira. He is twenty-six years old.

Zuckerberg cites “Minimalism,” “Revolutions,” and “Eliminating Desire” as interests. He likes “Ender’s Game,” a coming-of-age science-fiction saga by Orson Scott Card, which tells the story of Andrew (Ender) Wiggin, a gifted child who masters computer war games and later realizes that he’s involved in a real war. He lists no other books on his profile.

Zuckerberg’s Facebook friends have access to his e-mail address and his cell-phone number. They can browse his photograph albums, like one titled “The Great Goat Roast of 2009,” a record of an event held in his back yard. They know that, in early July, upon returning from the annual Allen & Company retreat for Hollywood moguls, Wall Street tycoons, and tech titans, he became Facebook friends with Barry Diller. Soon afterward, Zuckerberg wrote on his Facebook page, “Is there a site that streams the World Cup final online? (I don’t own a TV.)”

Since late August, it’s also been pretty easy to track Zuckerberg through a new Facebook feature called Places, which allows users to mark their location at any time. At 2:45 A.M., E.S.T., on August 29th, he was at the Ace Hotel, in New York’s garment district. He was back at Facebook’s headquarters, in Palo Alto, by 7:08 P.M. On August 31st at 10:38 P.M., he and his girlfriend were eating dinner at Taqueria La Bamba, in Mountain View.

Zuckerberg may seem like an over-sharer in the age of over-sharing. But that’s kind of the point. Zuckerberg’s business model depends on our shifting notions of privacy, revelation, and sheer self-display. The more that people are willing to put online, the more money his site can make from advertisers. Happily for him, and the prospects of his eventual fortune, his business interests align perfectly with his personal philosophy. In the bio section of his page, Zuckerberg writes simply, “I’m trying to make the world a more open place.”

The world, it seems, is responding. The site is now the biggest social network in countries ranging from Indonesia to Colombia. Today, at least one out of every fourteen people in the world has a Facebook account. Zuckerberg, meanwhile, is becoming the boy king of Silicon Valley. If and when Facebook decides to go public, Zuckerberg will become one of the richest men on the planet, and one of the youngest billionaires. In the October issue of Vanity Fair, Zuckerberg is named No. 1 in the magazine’s power ranking of the New Establishment, just ahead of Steve Jobs, the leadership of Google, and Rupert Murdoch. The magazine declared him “our new Caesar.”

Despite his goal of global openness, however, Zuckerberg remains a wary and private person. He doesn’t like to speak to the press, and he does so rarely. He also doesn’t seem to enjoy the public appearances that are increasingly requested of him. Backstage at an event at the Computer History Museum, in Silicon Valley, this summer, one of his interlocutors turned to Zuckerberg, minutes before they were to appear onstage, and said, “You don’t like doing these kinds of events very much, do you?” Zuckerberg replied with a terse “No,” then took a sip from his water bottle and looked off into the distance.

This makes the current moment a particularly awkward one. Zuckerberg, or at least Hollywood’s unauthorized version of him, will soon be starring in a film titled “The Social Network,” directed by David Fincher and written by Aaron Sorkin. The movie, which opens the New York Film Festival and will be released on October 1st, will be the introduction that much of the world gets to Zuckerberg. Facebook profiles are always something of a performance: you choose the details you want to share and you choose whom you want to share with. Now Zuckerberg, who met with me for several in-person interviews this summer, is confronting something of the opposite: a public exposition of details that he didn’t choose. He does not plan to see the film.

Zuckerberg––or Zuck, as he is known to nearly everyone of his acquaintance––is pale and of medium build, with short, curly brown hair and blue eyes. He’s only around five feet eight, but he seems taller, because he stands with his chest out and his back straight, as if held up by a string. His standard attire is a gray T-shirt, bluejeans, and sneakers. His affect can be distant and disorienting, a strange mixture of shy and cocky. When he’s not interested in what someone is talking about, he’ll just look away and say, “Yeah, yeah.” Sometimes he pauses so long before he answers it’s as if he were ignoring the question altogether. The typical complaint about Zuckerberg is that he’s “a robot.” One of his closest friends told me, “He’s been overprogrammed.” Indeed, he sometimes talks like an Instant Message—brusque, flat as a dial tone—and he can come off as flip and condescending, as if he always knew something that you didn’t. But face to face he is often charming, and he’s becoming more comfortable onstage. At the Computer History Museum, he was uncommonly energetic, thoughtful, and introspective—relaxed, even. He addressed concerns about Facebook’s privacy settings by relaying a personal anecdote of the sort that his answers generally lack. (“If I could choose to share my mobile-phone number only with everyone on Facebook, I wouldn’t do it. But because I can do it with only my friends I do it.”) He was self-deprecating, too. Asked if he’s the same person in front of a crowd as he is with friends, Zuckerberg responded, “Yeah, same awkward person.”

Zuckerberg grew up in a hilltop house in Dobbs Ferry, New York. Attached to the basement is the dental office of his father, Edward Zuckerberg, known to his patients as “painless Dr. Z.” (“We cater to cowards,” his Web site reads.) There’s a hundred-and-sixty-gallon fish tank in the operating room, and the place is packed with marine-oriented tchotchkes that Dr. Zuckerberg’s patients have brought him. Mark’s mother, Karen, is a psychiatrist who stopped practicing to take care of the children and to work as her husband’s office manager.