I wonder if the Dunbar number has been applied to online communities yet... On twitter you can follow thousands because there is no real discussion, its a stream of soundbites. On Facebook its people you know in real life and the number is inflated with old friends from school, distant family, acquaintances you rarely meet; the real number of people you connect with is likely much lower than your total friend number. On hubski it might be different (which is why i'm doing my little experiment) but then again maybe it isn't, only time will tell. I would say that there is a fairly central core of members here who post most of the topics and comments, they are familiar with each other and that leads to a real sense of community. I would guess that that core of members hold everything together but there is an upper bound on the number it can support before another core is needed to ensure the sense of community.