alt Hacker NewsFacebook only made sense in the context of living on a college campus… I think it doesn’t quite fit this pattern because realistically users were rarely in the “good fit” case for only ~4 years. Then, for each user, it becomes this sort of awkward slowly decaying network as people move on.
I agree with everything else you said though.
Facebook was once a friend network and they added more and more advertising and slop to it. If you were the frog in the pot as it heated, it feels normal to you that your feed is entirely made of spam. If you're still out of the pot now, it seems foolish to jump in, unlike when it was lukewarm.