I'm not a fan of downvotes in social media, whether it's here or youtube or anywhere else - because I think they are inherently anti-social - and hence counterproductive. However, I have used them very rarely. I don't envision changing that practice after HF21.
@dan once said that an upvote for one post is the same as a downvote for all others, so I prefer to do my "downvoting" by assigning rewards to others.
IMO, a positive value voting scheme that accurately appraises posts would be far superior - perhaps something modeled after a 2nd-price auction with reward curve modifications to penalize stake-splitting.
I know the consensus here is that downvotes are necessary, but I am not persuaded. To the best of my knowledge, we're all working from intuition because no one has ever done the analysis to identify the range of possibilities and determine if the benefits outweigh the costs.