The problem is humans are social creatures. So if all there is is Google, humans will do stuff on the internet and Google will know about it, but the social aspects of life remain offline and therefore inaccessible to advertisers. Facebook has managed to get people to spend a much larger percentage of their time online by moving part of their social lives there and they're gambling that a significant portion of that time people will do things that advertisers will be interested to know about.
Personally, I doubt this gamble will have the payoff they're apparently expecting. The primary focus during social interaction seems to be on, well, social interaction. Consumption only plays a very small role (though I'm sure Bacardi would like to pop up ads while you're chatting with your buddies on Facebook). Facebook basically catches people only when they're in the mood for fun and relaxation, whereas Google captures people right when they're in the mood for consumption and for spending money and to an advertiser that is much, much more valuable.