Disheartening to see that Facebook is unabashedly aping the cornerstones of various sites (reddit) and apps (Snap). That said, a downvote, or a "dislike" button, is almost always a double-edged sword - although it can be used to tune out hateful comments, it can also turn potentially meaningful comment sections into echo chambers extremely fast. I do not see this turning out well.
I always thought the only way downvotes buttons should be implemented is that they're non-functional, in that they do nothing other than let a user take an action; you could perhaps then influence what that specific user sees in the future, to help get an idea of their tastes, or instead not actually affecting where a comment or post shows up in an order.