Surely there's enough proof out there at this point that ranked comments do nothing to improve discussion, and that downvoting is always used as a "I disagree" button?
(1) I don't think that's the case, and
(2) Facebook's selling point to users has never been on being a neutrally optimized discussion site, it's been on algorithmic optimizing to show you what you want to see. Having a negative interest signal quite reasonably improved the data available for that optimization.
> and that downvoting is always used as a "I disagree" button?
Whether or not that is generally either true or desired by other sites implementing comment voting, given the nature and focus of Facebook, I absolutely do think that use of a dislike (not “downvote”) button for disagreement is perfectly consistent with Facebook's likely intent in implementing it.
Like Dislike Agree Disagree Happy Funny Surprised Sad Angry Winner Friendly Informative Creative Useful Optimistic Boring Old Bad Spelling
As well as whatever other options the site owner may have added. So there are systems out there which differentiate between a post someone disagrees with and one that's low quality, and they do make sure 'disagree' is not counted as a like or dislike on the database level too.
...which don't actually mean anything in practice because (1) disagreement effects perception of quality, so even honest voters in a system which distinguished the two will still be likely to mark a post that they disagree with as low wuality, and (2) people in practice, given the option, will mark a comment or post in the way that produces the effect they wish to happen to it, so if they want a post demoted because they disagree with it, and the forum offers marks for disagree which don't demote and some other mark which does demote, they choose the latter.
Maybe it stops holding up on Reddit scale though.
Additionally I think the best option is to completely hide the downvotes from other users, especially in the score numbers. Ideally you don't show the score to anyone but the comment owner.
On Reddit, once you dip below 1, people tend to simply vote down even more. Not because they disagree but because it's a bad comment. It must be, it's score is negative. From what I've observed, controversial comments (when marked as such) are usually the ones that have initially gotten enough upvotes to stay positive through the downvotes.
Hackernews also has (IMO) some signs of downvote trains where people downvote comments with the shadow but HN usually quickly hides the comment or dead/flag marks it to prevent further voting.
In an ideal system I would also not simply rank by the score or a variation of it. I'd spray in some controversial or low-ranked comments into the high ranking ones to ensure that people with not enough votes get visibility even when the discussion has been going on for a while.
Personally, I don't find anything bad about a downvote existing, it just needs to be carefully gated and manipulated so as to decrease the usage as a "I disagree" button without people wanting a "I disagree" button nor that vote brigading/raiding becomes a problem. Furthermore I think it's important that people don't have the score of a post so as to improve the neutrality of presentation.