For starters it is a very convenient laboratory to test social networking theories. Do current theories about social networks in the real world apply in the virtual world? What aspects of virtual social networks apply to the real world? Theories about those aspects that do apply can potentially be tested much more easily in the virtual world than the real world.
It's a great place to make predictions, for example, about anxiety. Do we know how social networks effect our health? Good place to try and differentiate between effects of face-to-face contact and friendship.
What can virtual social networks tell us about the real world? Can they be used to predict disease outbreaks?
People studying facebook for these kinds of things are usually not interested in facebook, but rather using it because it is a convenient way to examine social networks. Kind of like examining Drosophila in biology.