Facebook seems really considering to introduce a "dislike" button.
The Atlantic: Facebook Says It's Introducing a 'Dislike' Button
This anecdote has been told several times as an April Fool joke. But Mark Zuckerberg is seriously planning this simple but drastic change. He told that many users were waiting for the appearance of a dislike button. Indeed, "unlike" button to cancel your "like" has already disappeared, perhaps to avoid being confused.
Of course, the new dislike button will not be prepared for disgracing a post. Its use is to express empathy or sorry. For example, you would not "like" your friend's post in which he reported an accident. In this case, you can push dislike button to let him know you feel regretful about this tragedy.
Indeed, we sometimes wonder how to respond to bad news on Facebook. However, it is quite unsure whether the dislike button will work as Zuckerberg estimates. Many users may push dislike button to express their opposition to the post.
I think that Facebook has made significant efforts not to clarify the users' hatred. For example, you are notified when your friend request is approved, but you are never notified if your request is rejected. Similarly, you cannot know the fact your friendship was dismissed by the partner's decision immediately.
Social Network Service (SNS) can easily visualize your emotion to other people on the screen. If Facebook equipped some ways to express your negative emotion, every timeline would be filled with a considerable amount of grudge. Zuckerberg, whose goal is to make everything clarified, must be wise enough to know the negative aspect of SNS.
I think the introduction of "dislike" button is quite a risky attempt. As the author of the article I refer above, alternative icon, such as a heart mark, will be desirable. In this case, however, users will wonder which button is preferred when they see an agreeable post.
SNS is now an infrastructure. In my opinion, simpler interface is admirable.