What Facebook hasn't picked up on yet is that Social is "search" and "search" is Social. They are two sides of the same coin (pun intended). Search is a powerful economic engine because it marries people who are searching for something with people who want to sell it. But that is ultimately a singular experience, in our day to day lives we are ultimately affected by our peers in our commerce, we want to know what movies they like, what clothes they wear, etc. We want to fit into our group, its instinctual, and as folks have known forever that drives fashion and it drives commerce. So the economics of social are the same as search, except contextualized by your social graph.
By all accounts Google understands that and they are making good progress with G+, Facebook still doesn't quite get it and they are vulnerable. The only question in my mind is will FB figure it out fast enough to stay relevant, and if so can they execute on that understanding. If not they are toast and Google wins.
[1] http://www.google.com/finance?q=NASDAQ%3AFB&fstype=ii...