Gates took the top spot on the Forbes 400 list for the 19th year running, with a net worth of $66 billion. Zuckerberg, co-founder and CEO of the social networking giant, fell from No. 14 on the list to No. 36.
Investor Warren Buffett, the head of Berkshire Hathaway Inc., again took second with $46 billion, while Oracle Corp. (ORCL) co-founder Larry Ellison remains third with $41 billion. Brothers Charles and David Koch, co-owners of Koch Industries Inc., tied for fourth with $31 billion.
Rounding out the top 10 were Christy Walton and family, owners of Wal-Mart (WMT), with $27.9 billion; Jim Walton, Wal-Mart heir and CEO of Arvest Bank Group Inc. ($26.8 billion); Alice Walton and S. Robson Walton, also heirs to the Wal-Mart fortune ($26.3 billion and $26.1 billion, respectively); and Michael Bloomberg, founder of financial data company Bloomberg and currently the mayor of New York ($25 billion).
Forbes says the rich mainly got richer in 2012, with net worth rising for 241 members of its list and shrinking for only 66. Rising stock prices, a rebound in real estate values and rare art prices helped.