Getting a 'Dislike' would just be a complete buzzkill, which would discourage people from posting. I expect it'd just benefit a few trolls and people with an axe to grind, at the expense of everyone else.
Ever since they brought in more buttons for you to be able to interact with statuses on Facebook I have said there should be a dislike button but we still do not have one yet. I have also said the same about Twitter as well that we need a dislike button as well as an edit button but nothing yet.
I wouldn't mind a Dislike button. Not everyone deserves a trophy and the most of the reactions are just too close to being a trophy award for a post. It gives people a false sense of the true reactions to their opinions. A Disagree button might be better handled than the dislike?
I do not think you need a dislike button on the facebook. If you do not like the content, you can just move away without liking it. Why do you have to spread negativity?