Investor George Soros, at 81, has for the first time made the list of the 10 wealthiest Americans, Forbes Magazine reported in the 2011 edition of its annual rich list.
Less surprising, Bill Gates again topped the list with a net worth of $59 billion, up $5 billion. The Microsoft founder has been the wealthiest American every year since 1994.
Everyone in the top 20 saw their wealth rise in the past year with the exception of investor Warren Buffett, whose fortune fell by $6 billion to $39 billion as the value of his shares in Berkshire Hathaway dropped.
Mr Buffett has urged US president Barack Obama to tax millionaires more and has pledged to give away 99 per cent of his fortune to philanthropic causes.
Facebook founder Mark Zuckerberg was the biggest dollar gainer of the year. His net worth increased $10.6 billion since August 2010 to $17.5 billion as the private market valuation of Facebook nearly tripled to $66.5 billion. Mr Zuckerberg is now among the top 20 for the first time.
Coming in at number seven, Mr Soros had a fortune of $22 billion after adding $7.8 billion to his net worth thanks to investments in gold and related securities as well as equities. Soros switched to cash in the spring, allowing him to preserve his gains and miss market turmoil caused by the sovereign debt crisis in Europe.
This summer, Mr Soros announced he would close his hedge fund and give money back to outside investors, focusing on managing investments for himself and his family instead. Forbes said hedge funds are notoriously secretive and that after he shut his fund, Mr Soros shared more information with the magazine about his wealth.
"We always thought we were low on our estimates of him, but this time we had more details bringing him to number seven," said Forbes senior wealth editor Luisa Kroll.
Mr Buffet was number two and his declining fortune came as his Berkshire Hathaway stock fell by 9 per cent between August 25th, 2010, and August 26th, 2011, when Forbes compiled its data.
"He is also the most generous billionaire," Ms Kroll said of Mr Buffett, noting that $3 billion of his decline could be attributed to charitable contributions.
The Forbes list comes as Mr Obama has proposed higher taxes on millionaires so they pay at least the same rate as middle-class Americans, something he called the 'Buffett rule'.
Unlike Mr Buffett, Tea Party backers David and Charles Koch want Washington to tax them less as their cash pile increases. The brothers' wealth from conglomerate Koch Industries pushed them into fourth place with $25 billion each, each up $3.5 billion from last year.
In third place was Oracle founder Larry Ellison, whose wealth increased $6 billion to $33 billion.
Christy Walton, heir to Wal-Mart founder Sam Walton's fortune, dropped to sixth place at $24.5 billion despite a $500 million increase in wealth.
Casino owner Sheldon Adelson was in eighth place with a fortune of $21.5 billion, up $6.8 billion.
Jim and Alice Walton, also of the Wal-Mart family, were ninth and 10th at $21.1 billion and $20.9 billion respectively. Jim Walton's fortune increased by $1 billion while Alice Walton enjoyed a $900 million gain.
The full Forbes 400 list can be seen at forbes.com/Forbes400
According to the Forbes list of the world's richest people, published in March, the wealthiest Irish person is Indian-born businessman Pallonji Mistry who holds an Irish passport due to the Irish citizenship of his wife and children.
Mr Mistry fortune, gained in large part from his shareholding in the Indian Tata Group, is estimated at $8.8 billion, having grown from $5.8 billion last year.
Denis O'Brien comes next with $4.2 billion, an increase on last year's $3.5 billion.