@JeffVader: Maybe because Google returns a profit in the range of $20–25 billion USD and Facebook returns about $300 million USD.
Google already has a social network in Orkut and now in Buzz, so I would say that it is unlikely that they will buy Facebook any time soon.
While it doesn’t not make sense for Google to buy Facebook, it just doesn’t seem like the kind of move they will make.
A better strategy in the long run would be to create an ecosystem of products and services that surround social networks e.g. URL shortening services, video/photo sharing sites, social media and new aggregating services – primarily because social networks are not permanently popular (at least in the past social networks have come and gone as their popularity has grown and then been lost when the ‘next big thing’ has come along.) They’ve done a pretty good job of this already.