Nearly half of all Internet users in the United States belong to a hobby-oriented social network, a new study has found.
Plus, nearly a third belong to an online professional community like SWOM.
That's according to USC's Annenberg School and the
Center for the Digial Future (highlights from the report are in
this PDF).
Niche social networks will eventually grow into concentrated, powerful sources of word of mouth more than the big, Life magazine-like social networks of MySpace and Facebook.

Great Stuff! EMarketer is a great source of data. You should also take a look at Marketing Charts .
Do you