Facebook reached another milestone Tuesday: the social-networking site said it signed up its 250 millionth user.

Just five and a half years ago, Mark Zuckerberg invented the site in his Harvard University dorm room. Within months, he became the youngest self-made billionaire in history.

Zuckerberg's rise to Internet royalty is dramatized in Ben Mezrich's new book, The Accidental Billionaires. Mezrich charts Zuckerberg's transition from Harvard miscreant to Silicon Valley playboy — all while callously shedding himself of the "little people" who helped him on his way up.

"Mark Zuckerberg, after a particularly bad date, was home in his dorm room," Mezrich tells Guy Raz. "He was a sophomore, he was drinking some beers, and he hacked into all of the computer systems at Harvard, and he pulled pictures of all the girls on campus and he created a hot-or-not Web site where you could vote on who the hottest girl at Harvard was."

The malicious prank — aptly named facemash — ended up crashing Harvard's servers, and Zuckerberg was nearly expelled. But with the help of a friend, Zuckerberg turned the prank into the social networking giant it would become.

Mezrich never interviewed Zuckerberg (who in the end "opted out of talking to" the author). But he pieces together the story of Facebook through court documents, articles and interviews with his main source, Eduardo Saverin — Zuckerberg's spurned friend and original investor.

Mezrich dramatizes whole scenes where he details what "probably happened." He fends off criticism, denying Business Week's claim that the book is a "fictionalized account."

"There are a lot of journalists out there who don't quite get what I do or are frustrated by the way that I write. I write narrative nonfiction stories," he says. "It's an exciting way of taking a true story and opening it up for the readers ... It's certainly not fiction."

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