No. 8 -- Mark Zuckerberg, Facebook

NET WORTH: $6.9 billion

Zuckerberg made his big philanthropic debut a few months ago when he pledged $100 million to the Newark, New Jersey school system.

Zuckerberg's debut as a big-hearted billionaire garnered just about as much criticism as it did positive headlines.

Zuckerberg announced the donation on an episode of Oprah, but critics maintained that the more important entertainment trigger for Zuckerberg becoming a friend of philanthropy was the debut that same weekend of the movie, The Social Network which hardly paints the Facebook founder in the most positive light.

There was little doubt that Buffett and Gates wanted to trumpet the fact that the Facebook founder came on board with the effort, as Zuckerberg provided one of the few quotes in the press release from Giving Pledge about the newest big-hearted billionaires. Zuckerberg made clear in his statement, too, that just as Buffett and Gates are using Giving Pledge as a way to encourage all philanthropic giving from the richest global citizens, the Facebook founder's philanthropic turn is intended to encourage more members of the baby-faced billionaire set to start thinking about giving back.

"People wait until late in their career to give back. But why wait when there is so much to be done?" the co-founder, CEO and president of Facebook said in the release. "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."

Critics continue to snipe, not just at Zuckerberg's motivations for giving, but the entire concept of giving in such a public way. Skeptics say that the real trigger for these big-hearted billionaires is estate-planning, while the public trumpeting of their big hearts -- most recently associated with the controversial Facebook founder -- is as vain an effort as trying to amass more Facebook friends than the next person.

Philanthropic experts also question whether the Buffett/Gates pledge program is structured with any accountability. It's easy for a billionaire to pledge a majority of their fortune to charity, but who's minding the big-hearted billionaire store, and making sure that each and every one of the philanthropists follows through on the pledge when the time comes, and long after the positive press from the pledge has dissipated?

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