Okay, so I keep trying to wrap my head around this, but I just genuinely don't get this idea.
Facebook is huge, at the moment, but it has one public facing product. What does that product let you do? Be social, within its confines, and connect to everyone you ever knew, sharing links, photos and text snippets. Yeah, there's a lot of end-user value in that and, for as long as they can ride the power-law effects, Facebook will be an ad-revenue generating monster.
Google, on the other hand, has a suite of tools that lets people do almost everything else in the online world. Search is moving ever so slowly into the 3D/"real-world" through maps and product searches and now the goggles project (if it stands the test of time, of course).
In short, google helps its users manage their worlds whereas Facebook manages people in the Facebook world. Without significant expansion and diversification, Facebook has a much more precarious position, at least as far as producing end-user value goes.
Thus far, that innovation and expansion has been what? Facebook connect? Putting "like" buttons all over the web and tracking my every move? That's not value for the average Facebook user (for advertisers? sure, very much). So, what else? Letting me share all of my activities like what music I'm listening to, etc? All of that happens within the facebook walled garden – and that tactic makes the whole thing a bet the house gamble. As long as people stay with FB, it works, but once they drift away, it all falls apart just as quickly as it grew. (The power law can cut both ways)
I don't deny Facebook has a lot of value, but I just think that the more open framework/platform presented by Google is going to play out much more strongly in the long-run...