Consolidated communities like Facebook derived huge value because of network effects: everyone you want to talk to already has an account.

Smaller communities derive value from addressing the unique needs of their users, better moderation, the unique audience is the value. Each of those communities in aggregate may be more valuable than the previous, one-size fits all mega-network.

I'd expect (or maybe hope for) a stable point with a couple large general networks, and many small, niche gems.