On Tuesday, Facebook founder Mark Zuckerberg made major news when he and his wife, Priscilla, announced the birth of their daughter. But it wasn't just the touching letter to their kid that caught everyone's attention: Zuckerberg and Chan pledged to donate 99% of their Facebook stocks to improving the world for the next generation. (That's roughly $44 billion.) While the move should certainly be applauded, let's remember that Zuckerberg isn't the first billionaire to pledge to give away the majority of his wealth.Warren Buffet teamed with Bill and Melinda Gates back in 2010 to create the Giving Pledge , which basically asks the world's wealthiest people to donate at least half of their fortunes to charities and philanthropic causes during their lifetimes or in their wills. The pledge isn't a legally binding contract; rather, it's a moral commitment. For many of these billionaires, giving away half their wealth still leaves them and their heirs extremely well-off.So far, more than 138 people from 15 different countries have pledged to give away half their wealth. Here are some of the members of the Giving Pledge's billionaires list.