> I think there is no evidence of the mechanisms you describe.

Which part? If we are talking about this part:

>> I think that there's an element of validation and indoctrination that is a serious concern here.

Then I think there is evidence of that being true.

Take for example “incels”. They gather in online forums where they construct a worldview built entirely on misogyny, entitlement and hate. The ideas they are spreading among themselves are much worse than probably most of those people would come up with on their own. In their echo chambers they validate these ideas to one-another and make them seem acceptable.

https://www.ft.com/content/d71f3156-f94d-11e8-af46-2022a0b02...