I think there's something to this. Not sure if it's Dunbar's Number specifically, but something. It seems like there's a pattern where a community is nice and small, with people joining at a trickle. Things change when a jerk joins and starts doing jerk stuff. This tends to drive away the nice and helpful people, attract more jerks, and pushes people still there to be more jerk-ish themselves. Sometimes deft management can prevent it with some well-timed bannings, but I think this gets increasingly difficult as the community gets larger, and may be effectively impossible in internet-scale communities.