Facebook co-founder Mark Zuckerberg and Dustin Moskovitz have joined 15 other billionaires in a pledge to donate more than half of their wealth to charity, according to pledges released Thursday.

Zuckerberg, the 26-year-old chief executive and president of Facebook, said in his pledge that "people wait until late in their career to give back. But why wait when there is so much to be done? "

Zuckerberg has a fortune estimated at 6.9 billion U.S. dollars, according to Forbes magazine.

He was joined by fellow Facebook founder Moskovitz, American Online (AOL) founder Steve Case, investor Carl Icahn and junk bond pioneer Michael Milken.

The newcomers joined 40 U.S. billionaires who pledged to give over half of their money to charity in August. The philanthropy drive was organized by Microsoft founder Bill Gates and legendary investor Warren Buffett.

Except for the Gates family and Buffett, the initial group included Microsoft co-founder Paul Allen, New York Mayor Michael Bloomberg, pharmaceutical tycoon Patrick Soon-Shiong, Oracle founder Larry Ellison and Hollywood director George Lucas.

The wealthy families all participated in "the Giving Pledge," a donation drive launched by Buffett and Gates. The idea is to invite the wealthiest individuals and families in the United States to commit to giving the majority of their wealth to the philanthropic causes and charitable organizations during their lifetime or after their death.

A statement from the Giving Pledge website said "the pledge is a moral commitment to give, not a legal contract."