The "empathy" point is the real driver here. Everyone on Facebook has seen a dead pet/sick parent/horrible international tragedy/etc. post. Up until now, you

The "empathy" point is the real driver here. Everyone on Facebook has seen a dead pet/sick parent/horrible international tragedy/etc. post. Up until now, you either comment (effort) or "like," which feels wrong. I don't want to "like" your dog's death, so I do nothing. If I do nothing, that's less interaction on Facebook. If there's a third option — "dislike" — then I (and many others) will use it and provide more interaction on Facebook. I think +1 or "hug" is a better fit but regardless, it means a net-increase in interactions. More interaction by users is better for them. Surprised this didn't happen ages ago.