Facebook founder and chief executive Mark Zuckerberg and 16 other wealthy American families agreed to donate most of their fortunes to charity by joining a pledge started by billionaires Warren Buffett and Bill Gates.

The Giving Pledge, a group founded by billionaire investor Warren Buffett and Microsoft founder Bill Gates and his wife Melinda in June, said on Thursday the total number of American families taking the pledge has risen to 57.

“People wait until late in their career to give back. But why wait when there is so much to be done?, Zuckerberg, who is worth $6.9 billion according to Forbes magazine, said in a statement issued by The Giving Pledge.

 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.

Carl Icahn, an investor, former junk bond investor Michael Milken and AOL Inc co-founder Steve Case and his wife, Jean, will also take part in the project, the group said in a statement.

They join a group that already includes, besides Buffett and Gates, Microsoft co-founder Paul Allen, Oracle chairman Larry Ellison, David Rockefeller, CNN founder Ted Turner, New York mayor and Bloomberg LP founder Michael Bloomberg and Star Wars creator George Lucas.

Buffett, chairman of Berkshire Hathaway Inc., and Gates, 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,” he said.

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

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.