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.
Happens in real-life. Look at how many news stories you see about some speaker coming to a college and being shouted down (or food being thrown at them) so they can't speak.
A long time ago I read an interview with Charlton Heston. He said during the filming of Planet of the Apes, the costumes took so long to put on and take off that they left them on during lunch. One day he looked around and realized: all the humans were sitting together, all the gorillas were sitting together, all the orangutans...
