7. Mark Zuckerberg
7. Mark Zuckerberg
Hacker-in-Chief: Zuckerberg in 2006 in Palo Alto
Company: Facebook
Sales: $3.71 billion
Market Value: $75 billion-$100 billion (estimate)
Employees: 3,200
Advice: Embrace paranoia.

By the time Mark Zuckerberg celebrates his 28th birthday this May, Facebook will in all likelihood have gone public and become the biggest IPO of all time. The long-anticipated event will create hundreds of millionaires, result in a valuation of an Internet company that will approach $100 billion, and make the geek who dropped out of Harvard University his generation's Bill Gates.

Yet it has been only eight years since the social-networking site was launched from Zuckerberg's dorm room at Harvard. It would be easy to chalk a good bit of his success to luck and timing. But that would be a serious mistake. What's helped make Facebook the world's dominant social network is an obsessive entrepreneurial genius who has taken a page from another of Silicon Valley's legendary denizens, Intel's Andy Grove, who famously stated -- and lived by -- the dictum that only the paranoid survive.

Zuckerberg is the Valley's most paranoid entrepreneur these days, taking nothing for granted. It's why he has pushed out a constant flow of innovative changes to Facebook's platform, making it easier for developers to create applications for the community and ensuring that each new iteration keeps it ahead of the competition. It's the single most important explanation for why Facebook has yet to face any formidable rival in its space, including last year's challenge from heavyweight Google.

By John A. Byrne, contributor @FortuneMagazine - Last updated April 09 2012: 4:27 PM ET
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