He began to hang around with some of his newfound San Francisco friends, including night owl Jonathan Abrams, a programmer who had launched Friendster in 2002, in part to help people hook up. Parker was fascinated with Friendster, which that year became the world’s first online social network to grow its membership into the millions. But the service began sputtering when its systems could not handle the traffic. Parker, however, smelled an opportunity.

One day—in a scene fictionalized in The Social Network—Parker saw Thefacebook, as it was then known, on the computer of his roommate’s girlfriend, a student at Stanford. (In the movie, he gets his first peek after spending the night with a woman whose name he barely knows.) Parker had already concluded that the most promising way to launch a social network would be within a relatively closed community. College seemed perfect. He trawled the Web site and sent an e-mail to the kid who ran it—Mark Zuckerberg, then a sophomore at Harvard—suggesting that the two of them meet.

Matt Cohler, who joined Thefacebook shortly after Parker, is awed when he thinks about that pivotal e-mail. “Napster and Facebook are two of the most significant companies in the history of the Internet,” he says, “and in both cases Parker spotted them earlier than anyone—other than the people who invented them.”

Parker impulsively flew to New York, where he met Zuckerberg for dinner, and the two quickly bonded. A few months later, in June 2004, they ran into each other on the streets of Palo Alto, where Parker, unemployed (but still driving around in a BMW 5-series), was living with yet another girlfriend. Zuckerberg invited him to move into Facebook’s newly rented summer house. At first, Parker slept on a pad on the floor of co-founder Dustin Moskovitz’s room, until, as Moskovitz puts it, “things got more serious with my girlfriend, and I had to kick him out.” Perhaps more than at any other time in his life, Parker focused closely on one project.

Back then Parker apparently believed even more passionately in the company’s potential than did Zuckerberg himself. Peter Thiel—the billionaire hedge-fund manager and co-founder of PayPal, who became Thefacebook’s first investor—says that around that time “Sean consistently argued that Facebook was going to be really big. If Mark ever had any second thoughts, Sean was the one who cut that off.” In late August 2004, Zuckerberg and Parker went into a branch of Silicon Valley Bank to open a business account. With only two weeks before the fall semester, Zuckerberg was still talking about returning to Harvard, and the two argued over it right then and there, according to Ken Loveless, a senior vice president at the bank. Parker was adamant that Zuckerberg shouldn’t go back. (Zuckerberg dropped out.) Says Moskovitz, known for his dry humor, “Sean probably deserves less credit for turning Facebook into what it is than he thinks he does, but also more credit than anybody else thinks he does.”

Besides serving as Zuckerberg’s comrade-in-arms, Parker also worked to bolster his partner’s position, so what happened to Parker at Plaxo could never be repeated at Thefacebook. In the financing that Parker negotiated with Thiel, as well as a much larger deal signed seven months later with the Accel Partners venture-capital firm, Parker was able to negotiate for Zuckerberg something almost unheard of in a venture-funded start-up: absolute control for the entrepreneur. Because of that, Zuckerberg, to this day, allocates three of Facebook’s five board seats (including his own). Without that control, Facebook would almost certainly have been sold to either Yahoo or Microsoft, whose C.E.O., Steve Ballmer, offered $15 billion for it in the fall of 2007—only to be met with a blank stare from the then 23-year-old Zuckerberg.