One of the key driving factors behind this is that anyone can feel comfortable sharing something on Facebook. If you're a naturally shy person then there's no need to fear downvoting, the worst that can happens is that no one expresses a positive sentiment. You can make everyone aware of something you're doing or how you're feeling with only the possibility of a positive response. That's fantastic and it's what encourages people to share.
There is an argument that you can improve the quality of what people see by having a "dislike" or "downvote" option, like Reddit, HN or StackOverflow. The big difference is that Facebook is inherently personal. I don't mind the fact that someone might downvote me here or on Reddit but that changes when I'm talking about something incredibly personal to the people that are closest to me as is the case on Facebook.
I can see that they may introduce a way to offer a different sentiment, a dislike where the meaning is "I'm sorry this happened to you" or "It's bad that you feel this way". A way of actively stating you find something to be "bad" or "useless" content is never going to be present on Facebook though. Or at least, I can't see a positive outcome from it.