@AC: $5-6?
Your analysis only works if the business is sustainable, and will generate at least the same profits for maybe 10 years, and if the company actually does distribute those profits to shareholders. If all this is true, then your share of the business will eventually start to make real money for you.
In short, even if FB was at a normal 'fair' valuation, you'd have to be incredibly optimistic to buy in at that price, given that there's no protectable IP involved, and the entire history of the web makes it painfully obvious that the FB product will get up off it's collective arse and walk off as soon as it finds a greener field to chew its cud in.
Which only leaves one reason to buy in at a stupidly high price - a belief that the market will expand enormously. There's absolutely nothing to indicate that this will happen. There aren't even enough people on the planet to make it happen. And most of those currently left over are Chinese, and nobody in their right mind would expect them to flock to FB when they can do it their own way.