The Facebook phenomenon strikes again, this time helping catapult two new 20-somethings into the ranks of America's richest for the first time and bringing down the average age of America's richest ever so slightly to 65.7. Only eight American billionaires are under the age of 40, and three of them co-founded Facebook. The youngest? It isn't who you think it is. The world's youngest billionaire is now 26-year-old Dustin Moskovitz, who is eight days younger than his former Harvard roommate and Facebook co-founder Mark Zuckerberg. The social-networking site's first chief technology officer, Moskovitz left in 2008 and started Asana, a software company that allows individuals and small companies to better collaborate. The company has attracted several of the same early backers as Facebook and may one day be worth something. For now, though, Forbes estimates Moskovitz's entire $1.4 billion fortune comes from his 6 percent stake in Facebook. Zuckerberg may no longer reign as the youngest member of the Forbes 400, but he has bragging rights as the year's biggest percentage gainer -- his net worth jumped to $6.9 billion, up from $2 billion, making him worth nearly five times as much as Moskovitz and more than even Apple's. The third Facebook co-founder among the ranks is 28-year-old Eduardo Saverin, who once owned a one-third stake in Facebook. When Zuckerberg and Moskovitz 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 a co-founder bio on Facebook's site. Don't feel bad. We estimate that share makes him worth $1.15 billion. Technology, and in particular the Internet, has long been the best bet for getting rich at a young age. Bill Gates made his debut on the list in 1986 at age 30 with a net worth of $315 million. Michael Dell debuted at age 26; 19 years later, he is still among the list's 20 youngest. So too for Yahoo's (Nasdaq: YHOO - News) David Filo and Jerry Yang, who first made the ranks a dozen years ago. Indeed half of the 20 youngest America billionaires have made their fortunes in the tech industry, most via the Internet. Outside the world of the Internet, young rich-list members have been able to cash in from a few other industries such as finance and sports. Hedge fund manager John Arnold, 36, got his start as an oil trader for Enron in 1995. He is said to have earned $750 million for the company in 2001. When Enron collapsed a year later, he went into business for himself, founding Centaurus, a hedge fund focusing mostly on natural gas, energy trading. Five of the 20 youngest inherited their fortunes, including Scott Duncan, the only billionaire in his 20s who didn't strike it rich with Facebook. He and his siblings assumed control of the family's $12.4 billion pipeline empire after their father Dan Duncan's death last March. Worth noting about this bunch is not simply how quickly they've made their money, but how they are choosing to spend it, not so much on luxury homes or expensive toys but on causes about which they are passionate. Google's (Nasdaq: GOOG - News) Larry Page is 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 As for Zuckerberg, he still lives in a relatively modest rental home in Palo Alto, Calif. So what's he doing with his wealth? Not much so far, given that most of it is tied up in non-public shares of Facebook. Still, the 26-year-old announced on "The Oprah Winfrey Show" in September that he is giving away $100 million to Newark's schools. The gift is the largest philanthropic act by a person his age in American history.