Agreed, but I think this is a little beyond self-segration. If they just wanted to self-segregate, they'd find stories they agree with and digg them up, but extending that to also ruin other people's discussion is another thing. They are gathering with the explicit goal of drowning out other voices outside of their self-segregated group.

Perhaps another reason reddit isn't as susceptible is that it allows for user-created subreddits where like-minded folks can form such hives.