Dunbar's Number [1] puts a number of 150 as an upper limit for effective groups where everyone knows eachother - but these would presumably be active users, and it's not necessarily directly applicable to internet forums (but from my experience with irc and other communities, it does make some sense). I've been trying to start groups like this myself, but it isn't easy.
Yeah I think this isn't right for these types of communities because you don't actually want to know everyone. So that seems low. I'm wondering if we can find a decent upper bound.
That's a fair point. I also feel like the numbers of subscribers to a subreddit-style community is deceiving - the real number to be looking at is the number of people participating in the discussion, and the number of regulars. I'd be interested in seeing these stats for individual subreddits.
[1] https://en.wikipedia.org/wiki/Dunbar%27s_number