With a billion dollar valuation, the founders will have found ways to make themselves rich - http://news.cnet.com/8301-1023_3-57591017-93/snapchat-founde... - and if you already cashed out fuck-you money, then you might as well keep going and see where it lands.
If I was broke and offered $3b, sure. But if I had $10-20mil sitting in the bank… anything I do after that point is 100% risk-free.
Mind you, I'm really racking my brain on how this isn't all totally insane.