A lot of it boils down to different priorities at different ages. If I had started Facebook at 19 and Yahoo offered me $1 billion for it, I probably would have made the same decision Zuckerberg did and say that running Facebook is more fun than having $1 billion. If I founded Facebook today, I’d take the first offer that put >$10 million and run off to play with my kids, and you’d never have heard of Facebook again.