This is completely my view and is just a comment on the topic -
While many users love the idea of Facebook adding a dislike button, I don't think there are many users who are dying to have their own content disliked.
Disliking provides feedback to the poster which would generally discourage them from sharing. Liking provides feedback to the content poster, but it also provides a way to share good content with friends in their News Feeds. Disliking wouldn't result in anything being shared (because why would your friends want to see what you disliked?), so it would be a much more neutered functionality than liking.
Plus, Facebook would have to introduce the word "Undislike" to support undoing, which is just incredibly awkward.