The sort of person that faced with ownership of Facebook would say he would just retire and travel and dabble in philanthropy is also the sort of person who would sell out of a Facebook for $500m a few years earlier, $100m a year or two before that, $50m months before that...
Bill Gates is hardly "dabbling" in philanthropy, though. When you try to eradicate a major disease as the first step of your long-reaching plans, you get to use nicer words than "dabbling".
I wasn't intending to describe Bill Gates (although his fortune is currently $60b which is 60x what Zuckerberg apparently has in the bank and bigger than IIRC his holdings of Facebook), but most billionaires don't seem nearly as serious or careful about their philanthropy as Gates. Take Zuckerberg - he's not off to a good start with just donating millions to schools; that's not going to change anything like Gate's "let's eliminate polio forever" would.
