Facebook co-founders Mark Zuckerberg and Dustin Moskovitz this week pledged to join more than 15 other US-based billionaires and give away at least half their fortunes. The move comes as part of a philanthropic campaign started by activist billionaires Bill Gates and Warren Buffett. So far, nearly 60 billionaires have met the challenge.

"People wait until late in their career to give back," Zuckerberg said. "But why wait when there is so much to be done? 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."

At just 26 years old, Zuckerberg is worth almost $7 billion, making him the 35th wealthiest American. Moskovitz is worth $1.4 billion and is the youngest US billionaire.

AOL co-founder Steve Case also joined in the pledge. Other tech-industry billionaires who joined previously include Microsoft co-founder Paul Allen, Oracle co-founder Larry Ellison, and of course Microsoft co-founder Bill Gates. Other notables on the list include Michael Bloomberg, Carl Icahn, George Lucas, and David Rockefeller.

Gates, his wife Melinda, and Buffett started the campaign—called The Giving Pledge—in 2009 and made the effort public earlier this year. The organization doesn't collect any money or require the billionaires to do more than write a letter stating their intention to donate at least half their fortune to charity while they're still alive. The Giving Pledge is going to expand outside the United States soon, Buffett has said.