Dustin MoskovitzNet Worth: $1.4 billion
Source: Facebook
Age: 26
America's youngest billionaire is eight days younger than his former Harvard roommate Mark Zuckerberg. Facebook's first chief technology officer, he left in 2008 to start Asana, a software company that allows individuals and small companies to better collaborate. For now, his entire fortune comes from his 6 percent stake in Facebook.
Mark ZuckerbergNet Worth: $6.9 billion
Source: Facebook
Age: 26
The Harvard dropout and Facebook CEO was the biggest percentage gainer on this year's Forbes 400 list and the unwitting star of box-office hit "The Social Network." Last month he agreed to donate $100 million to Newark's troubled schools.
Eduardo SaverinNet Worth: $1.15 billion
Source: Facebook
Age: 28
Brazilian-born Saverin co-founded Facebook with Harvard classmate Mark Zuckerberg and for a brief time had a one-third stake. When Zuckerberg quit school to relocate to California, Saverin stayed behind to graduate. A year later Facebook sued him; he countersued. The parties settled with Saverin apparently getting a 5 percent stake and co-founder bio on Facebook's site.
John ArnoldNet Worth: $3.3 billion
Source: hedge funds
Age: 36
The former Enron oil trader founded hedge fund Centaurus after the energy outfit famously collapsed. His fund now has $5 billion under management, and he and his wife Laura recently pledged half of their wealth to the Gates-Buffet challenge.
Sergey BrinNet Worth: $15 billion
Source: Google
Age: 37
Emigrated from Russia at age 6; his mother was a research scientist at NASA. He met Larry Page in computer science Ph.D. program at Stanford and dropped out in 1998 to start Google. Brin now focuses on raising margins with Instant Search and building new businesses in communications, and he invests in space travel initiatives and Parkinson's research on the side.
Larry PageNet Worth: $15 billion
Source: Google
Age: 37
The Google co-founder dropped out of his Stanford Ph.D. program in 1998 to start the search engine. Nowadays his personal passions include buying up chunks of residential Palo Alto for a network of houses that use new types of fuel cells, geothermal energy and rainwater capture. He also rides a Zero X electric dirt bike and an electric sports car from Tesla Motors, in which he and Sergey Brin are investors. Page is a board member of the X Prize Foundation, a nonprofit looking for breakthroughs in genomics, energy and space exploration.
Daniel ZiffNet Worth: $4 billion
Source: inheritance, hedge funds
Age: 38
The youngest son of the late William Ziff Jr., who built the Ziff-Davis publishing empire (PC Magazine, Car & Driver, Boating) and later sold out. Daniel and his brothers Dirk and Robert inherited their father's fortune, which they have since reinvested in Ziff Brothers Investments. He recently got married.
Lorenzo FertittaNet Worth: $1 billion
Source: casinos, Ultimate Fighting Championship
Age: 41
Fertitta returns to billionaire status thanks to growing martial arts league Ultimate Fighting Championship, which he owns with his older brother Frank. Abu Dhabi investment is helping UFC go international. A scuba diver, he swims with sharks -- including tigers and great whites -- for fun.
Jerry Yang
Net Worth: $1.15 billion
Source: Yahoo!
Age: 41
Born in Taiwan, Yang moved to the U.S. at age 10. He created his Web directory in 1994 with partner David Filo when both were Stanford grad students. Yahoo! co-founder made his debut among America's 400 richest at age 29. Still with Yahoo!, but no longer its head honcho. He sits on other boards and makes a number of charitable donations.
