Facebook co-founders Mark Zuckerberg and Dustin Moskovitz have joined 15 other billionaires in a pledge to donate over half of their wealth to charity.

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

Zuckerberg has a fortune estimated at $6.9 billion, according to Forbes magazine.

He was joined by fellow Facebook founder Moskovitz and American Online (AOL) founder Steve Case among others.

The newcomers joined 40 US billionaires who pledged to give over half of their money to charity in August.

The philanthropy drive was organised 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 US to commit to giving the majority of their wealth to philanthropic causes and charitable organisations during their lifetime or after their death.