I have experienced this with groups. I like joining various facebook groups, because I think the general idea of them could help make facebook a healthy experience. The other day I joined what looked to me like a fairly "vanilla" philosophy group, and all the sudden every single suggested group was for a bunch of anarcho-capitalist-primitive-whateverism fringe nutjobs. It's actually very scary just how fast the algorithm adapts to pull you into the fringe echo chambers.