Powerball ticket pools make people drool, too. Companies like this are called unicorns for a reason-they don't exist (statistically speaking).
Your phrasing suggests that you compare Facebook (now) with lottery as a system. You should rather see Facebook as a lottery ticket that turned out to be the jackpot.
You can't go up to the lottery winner and offer to buy part of their winning ticket. You can buy shares of Facebook today (in fact, at a nice discount currently).