Carruthers was expounding on that very subject at GrogCon Three. He was, of course, in the bar, while, in the function rooms, the sercon fans were holding a panel on the exact composition of Chromatic Dragons, Spayed Gerbils, and Pangalactic Gargleblasters. I'd had my fill of arguments over the comparative merits of aging that old Janx Spirit in PVC or in Styrofoam coolers and had retreated to the sanity of the bar. The weak force that attracted him to the panel was overcome by the strong force attracting him to the bar. If it was universal we should note an increased concentration of fans in the bar, but it seems to be an equilibrium situation.
Individual fans diffuse in and out, keeping the average concentration of fannishness constant.
I wouldn't mind hypothesizing that fannishness is inherently repulsive, so you'll never get above a certain concentration without some external pressure, holding fandom together. Something like that. I mean, look at the atomic nucleus. The bigger it gets, the more neutrons need to be there to keep the thing from flying apart. That's why there are so many filkers, media fans and SCA types at worldcons, and even then the larger worldcons are hellishly unstable, like uranium nuclei.