[dragonwriter]

> Surely there's enough proof out there at this point that ranked comments do nothing to improve discussion

(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.