Facebook gets asked a lot why they don’t have a “Dislike” button. Mark Zuckerberg’s answer is that they don’t want to become a place where people have up and down polls.
He does realize that when someone shares a sad story (my dog died) nobody wants to be the one to click “Like”. So in answer to that, they are working on making an empathy button of some sort. This way you can let someone know how you feel and commiserate with them.
See his response to the “dislike” button here:
the Victor crew