Lucy Kinder at the Telegraph says:

Facebook is developing a 'sympathise' button as an alternative to the 'like' button.

If a user tags their status with a negative emotion, then his or her friends will be able to 'sympathise' with the post rather than press the 'like' button.

Of course, Facebook will never have a "dislike" button because Facebook is an advertising platform, and everything on Facebook has to be happy and like-able all the time. That's the problem with Facebook.