First, because I have lived in Gmail the last five years, there's loads of data in their that can make social networking even more powerful. Google will do a lot to mine these connections. This is just the beginning. But third parties will assist too. I love what Remail is doing by helping me easily find emails from contacts on my iPhone - even when I am offline.
Second, its agnostic. Google doesn't care which social network you join. If a user links their profile to their social graph, Google will hep you harness it.