For all of the positive things that might come from it, I think it’s safe to say this would absolutely happen:
“In fact, it’s pretty easy to imagine that happening – Facebook has already noted that users will commonly report content on the platform as ‘false information’ simply because they disagree with the premise of a story, or because they’re intentionally trying to target a particular publisher. And when you also consider the ways in which politically motivated groups have been misusing Facebook to sway voter opinions already, it’s easy to imagine a downvote option would be utilized for the same. “
Also, it would become an amazingly simple way to bully people, pages, groups, etc. Drowned out by the mob of down-voters, Facebook would in essence be silencing more voices than it already does. Do we really want that? Do we want the “crowd” to decide what is right and what is wrong? What should be on social media and what shouldn’t? Haven’t we seen enough of that as it is.