If you think Facebook is really really awesome, then let me ask you a question. How much would you be willing to pay to use it? I don't mean a monthly user fee, they aren't THAT stupid. But in terms of your attention and your 'desktop' space. How about a pop-up Ad? How about a pop-under Ad? How about both?
My point here is obvious.
In order to monetize this product they will need to take something from it's users. Very soon, the user count will stop expanding for Facebook, and when that happens, the only way they will show continued growth is by taking more from each user than they took before. As soon as they do that, the user base will shrink. This is just math.
I'm not buying Facebook. And if you're the kind of person who buys a stock because the CEO is in his 20's and wears a hoodie, then in my opinion, you're going to get what you deserve.