Tom Willmot of EdgeDesigns, one of our team lead developers posts on the launches of www.hotcars.com and www.dreamwheels.com last week. He succinctly sums up the niche social networks for those not familiar:
A niche social network is simply a social network aimed squarely at a niche interest, be it wine or geeks. You take the classic social network components: user profiles, friends lists, user messaging, forums and groups; and then you tailor the experience towards your intended niche. Throw in expert articles, regularly updated content and a strong multimedia element and there you have it.
(Note: I am a senior developer with NameMedia, and worked on these projects)