Agree with the sentiment of some on here.
A like is a pretty simple concept..... It's very easy for a reader (all readers)to determine why others like a particular post (be it cuz it's witty, or factual, etc...), but it would be way to to difficult to determine why someone dislikes a post.
Motives could range from "I think you are wrong", "Dude, you are not funny", "This post has horrible grammar" or "We had an argument 6 months ago, so I just don't like you and will continue to dislike everything you say on principle".
For the everyday reader, when I see "likes" I tend to tune in more, because, there is likely good info in there or something witty to make me smile. A dislike would probably do the same thing, except make the reader have to try to discern why a post was "disliked". They'd probably guess wrong 90% of the time.
Way easier to do it the old fashion way...... Quote what you dislike and then blast em'