Any social dynamic seen in the real world can, and generally does, happen here, at least occasionally. I can name some historical, and a few current things that probably count, but the people involved would probably not like if if I did that.
At one time, we had a "like"-based reputation system. Clique behavior was seen within it. Circles of people who always liked each other's posts drove their reputation up above the masses because of the focused shared behavior.
When you have persistent users, people get to know each other. Knowing each other "in real life" isn't required.