I think
if there's a way to make it work, it would have to involve restricting the sizes of individual communities—probably with a "soft limit" and a "hard limit". Above the "soft limit", people can't join independently, but people can still be invited by friends; above the "hard limit," people can't join at all.
Based on my understanding of current research on the topic, I'd say to put the "hard limit" at about 150 members, and the "soft limit" at somewhere between 75 and 100 (because people will really want to bring in their friends; in fact, that's a major part of the point, and an unqualified positive for the community!).
I can also say from my own experience of running a small browser-based game that is almost 100% driven by player-to-player interaction, you need a critical mass of people interested in active discussion to keep such a group going, but if you have too many the volume of messages becomes offputting and it's hard for new members to feel comfortable speaking up. ...And that it's really, really easy for small, vocal, aggressive cliques of people within a given group to ruin things for the rest.