Facebook doesn't make much sense. The like button means three things:
I like this
I want to be notified when people comment on this
I want other people to see this
It stinks when something bad happens to a friend and they post about it, for sympathy, "I lost my job", "My dog died", etc, well, it stinks for my friend when those things happen. But it also stinks for me, because I want all of my friends to know, so they can show sympathy, and I want to be notified when other people say nice things to my poor, jobless, dogless friend. I just don't want to click the like link, because I don't want people to think that I like it that my friend is living under a bridge without a dog. This is why we should have a dislike* button, it would differ from the like button in name only.
*It could also be the "I express my sincere condolences" button.