There are several niches I would like to start a social network in..
I find they can bridge the gap when it comes to finding others with interests similar to your own in an otherwise isolating environment.
The two challenges still facing me are the software & knowledge of 'how to get it started'.
In any case, these will very likely become popular in thousands of niches, just as forums and information websites have.
Hopefully an opensource implementation arrives before the crest peaks.