I stumbled across a social network just for academics (no, I don't own it or get any commission for this). Just google academia.edu and you'll find it. The basic premise is that it's Facebook for Academics who can follow each other and read the academic papers they write. It serves not only to connect people with the same interests but also as an alternative publishing platform. It's pretty cool stuff. That got me thinking; if it can work for academics, what other niches are out there for social networks. Facebook and Google+ have covered the 'everyone' category already, and myspace has music. One that I think could work would be a Comedian social network (they share jokes, post their clips onto their profiles, find and communicate with their following etc). What other social network niche ideas do you think you would use yourself?
Basically anything. Have a look at Buddypress.org. You can create any kind of social network. If you like movies...create one for movies. Same with sports, etc, etc.
See, you can develop an active community related to any niche... It completely depends on you, how you are making it active. People love to participate into any community, provided its informative and merging more than one niches together. Another thing that audience hate in a community is blatant promotion and spam.