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http://archive.org/download/shortfunk/sfmaintainingapragmaticgroupoffriendsonfacebook.mp3
It has been said before that there may be an optimal community size, and something like the Robin Dunbar observation of 100-150 seems common. Beyond that it's mostly just entropy or you run up to the physical limits of how much you can read and write in a given time span.
If you're designing for federation or self-hosting then maybe optimizing around this sort of number is reasonable, and I expect that it happens already anyway but without explicit knowledge.