(Warren Buffett, Bill Gates billionaire charity story updated to reflect Facebook's Zuckerberg pledge, 17 new Giving Pledge billionaire members)

NEW YORK ( TheStreet) -- Berkshire Hathaway ( BRK.B) CEO Warren Buffett and Microsoft ( MSFT) founder Bill Gates are staples of the Forbes annual ranking of the world's billionaires.

In the latest Forbes ranking, the two icons of American capitalism finished No. 2 and No 3. Notably, both men fell behind Mexico's telecom titan, Carlos Slim, mostly because of the Oracle's hefty charitable pledges. Slim, on the other hand, has been criticized in the past for his relative lack of giving. Now Buffett and Gates created their own billionaires list, ranked according to those who confer a majority of their fortunes to charity.

Buffett and Gates haven't give up their spots as the two richest Americans, according to the new Forbes 400, released on Sept. 22. Gates is No. 1 among the richest Americans, with a net worth of $54 billion, and Buffett was No. 2, with a net-worth of $45 billion.

Buffett and Gates -- whose fortunes were linked when Buffett agreed to donate a majority of his wealth to the Bill and Melinda Gates Foundation -- have been on a campaign called the Giving Pledge this year. The two men hope to "soft sell" their way to as much as $600 billion in pledges, to use the Oracle's phrase. Still, only three of the top ten wealthiest on the Forbes global roll were among the 40 big-money donors on the original Buffett and Gates big-hearted list. Who were they? You guessed it: Buffett and Gates were two of the three.

Larry Ellison of Oracle ( ORCL) was the third member of the global top ten richest to join Buffett and Gates. Meanwhile, only two members of the Top 10 among the Forbes 400 Richest Americans, specifically, joined Buffett and Gates in making the big charity pledge when it was first announced: Ellison and Michael Bloomberg. Ellison is No. 3 on the Forbes 400, with an estimated wealth of $27 billion. Bloomberg rounded out the Forbes 400 Top 10 with an estimated wealth of $18 billion.

Buffett and Gates also plan to spend more time lining up the internationally based billionaires. So far, there's not one member of the GivingPledge group from outside North America.

Gates and Buffett have been busy over the past several months trying to line up billionaire philanthropists in China and other emerging markets, an effort that not only included a heavily covered billionaire dinner in China but led to its own CNBC special.

The results from the emerging markets effort are yet to be revealed, but Gates and Buffett's Giving Pledge announced on Wednesday that 17 more of the richest Americans have pledged a majority of the their fortune to charity.

The new group of big-hearted billionaires includes corporate raiders Carl Icahn and Michael Milken, Internet pioneer Steve Case of America Online, hedge fund and private equity luminaries like Leon Cooperman of Omega Advisers and Ted Fortsmann of Forstmann Little, as well as heirs to the Cargill fortune.

However, the new member of the Gates/Buffet big-hearted billionaire group garnering most of the attention is Facebook founder Mark Zuckerberg. Zuckerberg's colleague Dustin Moskovitz also joined the big-hearted billionaires group. Together, the two social networking pioneers have a fortune of $8.3 billion, according to the most recent Forbes ranking of the Richest 400 Americans -- Zuckerberg ($6.9 billion); Moskovitz ($1.4 billion).

All told, the 17 new members of the Buffett/Gates Giving Pledge group have a combined fortune estimated by Forbes of at least $36 billion. Not at the level of Gates or Buffett's individual fortunes even when amassed, and only two of the new 17 big-hearted billionaires make it into the Top 10 among the Giving Pledge group -- Zuckerberg and Icahn -- but hey, every billion counts as a drop in the big-hearted billionaire bucket.

In any event, what follows is a list of the 12 richest Americans -- excluding Buffett and Gates -- who have pledged a majority of their fortunes to charity, with the latest data from the just-released 2010 Forbes 400 ranking of the richest Americans.

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