I think the biggest problem is when we're all served a uniquely personalized feed. Everyone on Hacker News gets the same front page, but on Facebook users get one specifically tailored to them.

If Hacker News filled their front page with hate speech and self-harm tutorials there would be public outcry. But Facebook can serve that to people on their timeline and no one bats an eye, because Facebook can algorithmically serve that content only to people who engage with it.