There are tons of niche social networks available, and a lot more than are still popping up. Here's just one list of a few of them, and there's hundreds more. If you are an entrepreneur that is planning on building one or growing one, you should know something that might help your endeavor. People have become immune to the phrase "social network" and don't really give a shit about joining your new one just because it has x feature.
What is a niche social network? It's a community of people that access the platform around a specific topic, maybe to learn about it, maybe to share - whatever. That's irrelevant. The point is that they gather, they communicate, and they share - this makes it an online community. What's better about the word community? It's inert. It doesn't have that irritating web2.0 buzzword egoism attached to it. The word also portrays utility, so people actively explore them. It's also been around much longer.
So here's the thing, stop calling your niche social network a social network and call it a community instead. If your recipe-centric website like www.recipefy.com is about cooking and sharing recipes, call it a community because it is exactly that, and people won't feel defensive about joining it. They won't consider it as much of a burden signing up just to be on yet another stupid, fad social network.