Facebook applies some magic to decide which posts show up in your feed and which comments show up at the top of those posts.

When a metric like “engagement” is optimized for, this seems to result in controversial/divisive content making its way to the top. So yes, Facebook is a medium, but it is a medium that encourages certain content by rewarding it with more eyeballs and interaction than other content.

As a contrast, Reddit’s default sort is “best” which tends to put higher scored comments towards the top. Users have the choice of sorting by controversial, but that is transparent to the user that that’s what they’re getting. Reddit’s downvote lets users effectively shun content in a way that Facebook doesn’t - this has its trade offs, as it seems to encourage groupthink and echo chambers.