> A site that optimizes for content that people heavily interact with (engagement) is different than a site that prioritizes the relationship type between users

I think that's hit the nail on the head. In a neutral environment, a user would be shown a feed of friends posts with ads mixed in for their demographic.

In an optimised for engagement environment, a user is shown posts that are known to increase engagement and quite often those posts are inflammatory.

This engagement environment is much more likely to expose people to extreme views