Facebook likely has a watson-like technology that can answer a lot of interesting questions about a user, but "understanding emotions"... that's still a ways off. Maybe a long long way off. But it's wrong not to be very, very concerned.

Suffice it to say: (Paraphrasing Dijkstra) Whether Facebook understands human emotions from text is as relevant a question as whether a submarine can swim. I think they can get a helluva lot out of what you're typing and what you're liking. That should be enough to worry about.