Warren Buffett's estimated fortune dropped by $6 billion over the past year to $39 billion in the just-released Forbes 400 listof richest Americans.  He's the only person in the top 20 to see his wealth drop.

He's still number two on the Forbes list, but the gap between him and his friend Bill Gates has widened.  The Microsoft Chairman's wealth is estimated at $59 billion, up $5 billion from last year.

That puts Gates $20 billion dollars ahead of Buffett.

Oracle's Larry Ellison is catching up with Buffett.  He's number three with an estimated $33 billion, $6 billion more than last year.

One reason for Buffett's fading fortune is a decline of about 17 percent in Berkshire Hathaway stock over the past twelve months.

But there's another big factor:  Buffett is giving his money away.

With this year's donation of $3.27 billion to the , he's the big winner on the Forbes list of the "biggest charitable gifts made by rich listers in 2010 and 2011."   Over six years Buffett has donated Berkshire Hathaway stock worth almost $10 billion to the Gates Foundation.

Hedge funder Steven Mandel (#293 overall) is number two with his donation of $134 million to his Zoom Foundation.

Current Berkshire stock prices:

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