I think we should be fascinated by the endurance of niche social networks.
They were here before the social media explosion, they're still going strong, and they will be here after it washes away.
The successful niche online communities you see today aren't about to be swallowed up by Facebook, Twitter, Google+, LinkedIn and Pinterest.
Successful communities has very little to do with technology and a lot to do with developing a strong group identity. For many niche communities, using an older platform is part of their group identity.
Better platforms don't create better communities.
You should worry less about social media platforms and more about how to create a unique group identity that other people set out. That involves a strong history, clear group symbols, elements of ideology and clear, stated, beliefs.
Easy to say, of course, much harder to do.