>>Is it the like/upvote mechanism?

Yes. This is what created the validation feedback loop, and what generally causes the social harm

The like button, and the variations on it is IMO the biggest problem with social media

Forums and other discussion boards did not have these problems and while you could have "flame war" that involved actual argumentation back and forth not 1000's of passive visitors choosing to upvote, or downvote on a post with no argumentation