Facebook founder Mark Zuckerberg is among 17 US billionaires taking the pledge to donate the majority of their wealth to charity, organisers of the effort said on Thursday.
The project launched this year by the two richest Americans, Warren Buffett and Bill Gates, said a total of 57 individuals or families had now signed the Giving Pledge.
"People wait until late in their career to give back. But why wait when there is so much to be done?" said Mark Zuckerberg, who is chief executive and president of Facebook and has a fortune estimated by Forbes magazine of 6.9 billion dollars.
"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."
Steve Case, founder of the onetime Internet giant AOL, was also among the latest group to join the pledge.
"We share the view that those to whom much is given, much is expected," Case said in a letter with his wife Jean.
"We realize we have been given a unique platform and opportunity, and we are committed to doing the best we can with it. We do not believe our assets are 'ours' but rather we try to be the responsible stewards of these resources."
The Giving Pledge is the brainchild of Microsoft mogul Gates and investment guru Buffett who want to convince the richest people in the country to give 50% or more of their fortune to charity.
New names in the group include investor Carl Icahn and former junk bond guru Michael Milken.
The initial group included CNN founder Ted Turner, New York Mayor Michael Bloomberg, Oracle co-founder Larry Ellison and Hollywood director George Lucas, as well as Buffett and Gates.
Buffett said they simply started working their way through Fortune magazine's list of 400 US billionaires, who between them have an estimated net worth of 1.2 trillion dollars.