Silicon Valley is populated with people who got lucky once and cashed out, usually thanks to the largesse of a free-spending major corporation like Google, Microsoft or (now) Facebook. Many simply disappear with their winnings and are never heard of again. Others return with new ideas and new companies. Why? Partly because that’s the only thing they know how to do — and partly because they want to prove that they weren’t just fortunate, they were good. And that’s the challenge facing this latest crop of success stories, too. How they fare remains to be seen.