(Bloomberg News) Facebook founder Mark Zuckerberg and 16 other wealthy U.S. individuals and families agreed to give most of their fortunes to charity, joining a pledge started by billionaires Warren Buffett and Bill Gates.

Investor Carl Icahn and AOL Inc. co-founder Steve Case will also participate in the Giving Pledge project, the group said today in a statement distributed by Business Wire.

The number of participants has risen to 57 in the U.S. since the project was announced in June. Buffett, the chairman of Berkshire Hathaway Inc., and Gates, the chairman of Microsoft Corp., are also expanding the philanthropic effort by meeting with billionaires from other countries, the statement said.

"In just a few short months we've made good progress," Buffett said in the statement. "The Giving Pledge has re-energized people thinking about philanthropy."

The project aims to persuade billionaires to commit more than half of their wealth to philanthropy, according to the statement.

"People wait until late in their career to give back," said Zuckerberg, who is worth $6.9 billion according to Forbes magazine. "But why wait when there is so much to be done?"

New Donors

Individuals and families who joined this month also include Michael and Lori Milken and Dustin Moskovitz of California; Joe and Rika Mansueto of Illinois; Nicolas Berggruen, Ted Forstmann, Sidney Kimmel, Tom and Cindy Secunda, and Charles Zegar and Merryl Snow Zegar of New York; Lee and Toby Cooperman and Duncan and Nancy MacMillan of New Jersey; David and Barbara Green of Oklahoma; Denny Sanford of South Dakota; and Lyda Hill and George P. Mitchell of Texas, according to the statement.

"High net-worth donors rely on their peers when they make decisions about their philanthropy," Una Osili, director of research at the Center on Philanthropy at Indiana University, said in a phone interview.

Among previously announced participants are Microsoft co- founder Paul Allen, Bloomberg LP founder Michael Bloomberg, filmmaker George Lucas and Turner Enterprises Inc. Chairman Ted Turner.

Tom Secunda, Charles Zegar and Duncan MacMillan are founding partners of Bloomberg LP, the parent of Bloomberg News.