Funny thing is one of my favorite forums uses up votes and down votes. Conversations are much more civil and on topic. Rather than showing them separately. It is an overall tally. If someones comments hit an overall -5 they are automatically hidden. So, anyone spewing nonsensical rhetoric, &c. Get wiped away from view. But you have the option to expand and view the comment if so desired.
Just think of here. All the constant trolling, Apple bashing comments, what would Steve Jobs do or complaints about products which have nothing to do with an article would disappear. Like an iPhone rumor article then people complaining about the lack of updated Macs would be down voted into oblivion or Tim Cook doing something on his free time filled with complaints about lack of focus on the Mac. I like MacRumors but it gets very negative. Perhaps it is nostalgia but I remember it being much more civil and enthusiast filled ten to fifteen years ago.
Down votes might actually help with Twitter and Facebook civility. Then people might see that their vile comment which gets 100 likes actually gets 10,000 dislikes or their comments are hidden from the conversation unless someone chooses to expand it. At any rate it might encourage at least some to keep a civil tongue. Especially if friends, family, colleagues and those they admire down vote them.
Unfortunately, there is also the issue of what is being down voted. If some article about a horrible event is linked. Are people down voting your comment or the tragedy?