The October 2007 Nielsen Online numbers for October reflect substantial year over year growth for Facebook in the U.S. compared to MySpace, which is no surprise. LinkedIn had significant growth as did Club Penguin and Buzznet.com from their smaller bases.
At this point, even with the unique user stats offered by Nielsen Online subject to question, for the general purpose social networks in the U.S. MySpace and Facebook are the main adult show. Windows Live Spaces has had growth via the monster Microsoft engine, but two other giants are waiting in the wings. Google, with about 120 million unique users, and Yahoo with 110 million, according to Nielsen Online, are expected to user their registered user bases (Yahoo claims 250 million email users worldwide) to fabricate new social networks with learnings from the two current giants.
In the next few years, social networking concepts will be woven into the fabric of practically every major application and Web service.
The challenge for users will be rationalizing all the social network spheres to which they belong. But it's a known issue--humans have been dealing with it for a few millennium but not at this scale.
Max Ernst: "Young man intrigued by the flight of a non-euclidean fly"