An additional 17 U.S. billionaires, including Facebook co-founders Mark Zuckerberg and Dustin Moskovitz, have pledged to give away at least half their fortunes in a philanthropic campaign led by Warren Buffett and Bill Gates.
A total of 57 billionaires now have joined the Giving Pledge, which was launched by Microsoft founder Gates and investor Buffett in June.
Gates, his wife, Melinda, and Buffett have asked U.S. billionaires to give away at least half their wealth during their lifetime or after their death, and to publicly state their intention with a letter explaining their decision.
The Giving Pledge does not accept money or tell people how to donate their money but asks billionaires to make a moral commitment to give their fortunes to charity.
"People wait until late in their career to give back. But why wait when there is so much to be done?" Zuckerberg, who gave $100 million in September to the beleaguered public schools of Newark, N.J., said in a statement.
"With a generation of younger folks who have thrived on the success of their companies, there is a big opportunity for many of us to give back earlier in our lifetime and see the impact of our philanthropic efforts," he said.
In addition to Zuckerberg and Moskovitz, the world's youngest billionaires, pledges were made by AOL co-founder Steve Case, financier Carl Icahn and Michael Milken, a former Wall Street executive who went to prison in the early 1990s for securities violations.
Forbes magazine said the United States is home to more than 400 billionaires, the most of any country.
Individual Americans gave more than $227 billion in 2009, according to the Giving USA report by the Center on Philanthropy at Indiana University, down just 0.4% from the previous year despite the U.S. recession.
Buffett pledged in 2006 to give away 99% of his wealth to the Bill & Melinda Gates Foundation and family charities. Bill and Melinda Gates have donated more than $28 billion of their fortune to their foundation.