On a semi-related note, since we’re talking about scaling and breaking communities.
In anthropology there’s this term called Dunbar’s numbers. Briefly:
[Dunbar] proposed that humans can comfortably maintain 150 stable relationships
Is it possible within Discourse to find out what the average size is of the circle of users one user interests with? And could we draw conclusions from a general analysis of user groups, custom groups, etc.?
The reason why I ask is because: perhaps hitting the limits of or breaking these supposed limits of a social sphere might be a contributing factor in a community’s breaking points. 
Is that something Discourse can analyse at scale? It would make for an interesting study.
Edit: previously, the theory of Dunbar’s number was also mentioned in this older discussion, albeit in a different context. Don't be fooled by activity metrics – they may vary greatly in different community types - #8 by HAWK