Is that unexpected though? Or even a necessarily negative thing? I don't think so - especially in the case of a political ideology discussion forum like /r/libertarian, most of the people who went there to seek it out and subscribe to it, more likely than not, share those beliefs.

What is the difference between describing something as extreme group think, and describing it as a circle jerk? How does one differentiate between this being a valid description, and an unwillingness to accept that one's position is the unpopular one?