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.

[1] https://en.wikipedia.org/wiki/Dunbar%27s_number