Yeah, I belong to a bunch of fic prompt comms that are like that -- going back to structure, they tend to post the same kind of thing, daily, and that kind of consistent activity does tend to build up. But at the same time it puts all the pressure on the mods to keep things moving. I do remember that happening after a while on LJ with even busy comms, though -- people lose interest, or burnout happens, or whatever. I'd be almost interested in how subredditors keep stuff going, but....reddit.