Good stuff:
Niche Social Networks are not Businesses
Niche Social Networks are not Businesses
You see which way it flows? With a niche business, the social network is a subset of the functionality if it exists at all. If it is just a niche social network, it probably is not a business.
Once the social network qua social network is healthy, you can then monetize it. It's a lot of work, and the financial rewards are little, but if you do it right, you can get a strong and loyal user base and maybe make a few friends.