Is Facebook a network of networks?

Beyond Facebook, there are several other sites I use frequently. Google, obviously, is necessary for when I want to search and email other people. Last.fm has slowly become my choice for listening to music and finding new bands. And recently, I joined Streamy for reading newsfeeds. These sites are all networks: usually, Last.fm is the only place I need to go to for music information. And I’ve never had to resort to a search engine past Google for most of my general needs.