Microsoft founder Bill Gates topped the Forbes magazine list of the 400 richest Americans on Thursday as Facebook's Mark Zuckerberg leapfrogged Apple's Steve Jobs in the rankings.

Gates, 54, who has stepped down from day-to-day activities at Microsoft to concentrate on philanthropy, has an estimated net worth of 54 billion dollars, according to Forbes.

Berkshire Hathaway's Warren Buffett, 80, was the second-richest American with a net worth of 45 billion dollars followed by Oracle's Larry Ellison, 66, with 27 billion dollars and Walmart's Christy Walton, 55, with 24 billion dollars.

Also in the top 10 were Charles Koch, 74, and his brother David Koch, 70, of Koch Industries, each with an estimated net worth of 21.5 billion dollars.

Other members of Walmart's Walton family were next: Jim Walton, 62, with a net worth of 20.1 billion dollars, Alice Walton, 61, with 20 billion dollars and S. Robson Walton, 66, with 19.7 billion dollars.

New York Mayor Michael Bloomberg, 68, founder of the Bloomberg financial news agency, rounded out the top 10 with a fortune estimated at 18 billion dollars.

Google co-founders Larry Page and Sergey Brin, both 37, were next, each with a net worth estimated at 15 billion dollars.

Technology titans occupied four more slots in the top 20: Michael Dell, 45, founder of Dell Computer, with 14 billion dollars, Microsoft chief executive Steve Ballmer, 54, with 13.1 billion dollars, Microsoft co-founder Paul Allen, 57, with 12.7 billion dollars and Amazon's Jeff Bezos, 46, with 12.6 billion dollars.

Facebook's Zuckerberg, 26, ranked number 35 on the Forbes list with a net worth of 6.9 billion dollars, up from two billion dollars last year.

Other notable figures on the list included News Corp.'s Rupert Murdoch, 79, at number 38 with a net worth of 6.2 billion dollars and Apple's Jobs at number 42 with 6.1 billion dollars.

EBay co-founder Pierre Omidyar, 43, ranked 47th with 5.5 billion dollars followed by Google chief executive Eric Schmidt, 55, with 5.45 billion dollars.

New York real estate tycoon Donald Trump, 64, ranked number 153 with a net worth estimated at 2.4 billion dollars.

earlier related report
Facebook out to make mobile phones more social
San Francisco (AFP) Sept 22, 2010 - Facebook sees a promising future in mobile phones but the online social networking star is not building its own handset, the founder said in a TechCrunch interview posted online Wednesday.

"Our goal is to have Facebook be everywhere and everything be social rather than a specific device," founder and chief executive Mark Zuckerberg was quoted as saying.

"We're not trying to compete with Apple or the Droid (mobile phone from Motorola) or any other hardware manufacturer for that matter."

Facebook arranged for the interview to dispel speculation that the Palo Alto, California, company was working on a mobile phone.

Zuckerberg told TechCrunch that rumors may have resulted from mistaken interpretation of Facebook's efforts to infuse its services and features in the gamut of mobile devices.

"Our goal is to make it so that we can design the best integrations in the widest variety of phones," Zuckerberg said, stressing that Facebook was not building a mobile operating system or hardware "from scratch."

Software can make mobile phones more personalized and social by letting people sign in the way they do to customized home pages at Google, Yahoo! or elsewhere on the Internet, according to Zuckerberg.

"Just make it so that you log into your phone once, and then everything that you do on your phone is social," he said.

"I guess maybe Google or Microsoft could log you into the browser, but we can't because we don't build a browser," Zuckerberg continued. "But, that is the basic strategy."

Facebook is trying to be the platform for a "social layer" in all Internet-linked devices, according to the founder.

Facebook is investing heavily in weaving its software into iPhones since the Apple smartphones have a commanding presence in the market and has been increasing focus on handsets powered by Google-backed Android software.

"If Windows Phone 7 takes off, I'm sure we'll put resources into that," Zuckerberg said.

Microsoft's latest-generation mobile software platform is to be released next month.