I agree with most comments here - just ignore it / laugh it off.
Or at least convince yourself and others that you are.
Once that is done, see if it does have any merit. Is it what could have been a constructive comment, disguised after having rolled in mud? If it read "his / her last lecture was so boring that my table slept off" - just laugh it off but see if you could make it more interesting next time. Or just ask the class for their suggestions (smilingly and not angrily), saying that you read that comment on fb.
It is interesting how good some inputs from students can be.
Or if the student just said it out of spite - well, ignore it and dont see their fb posts. Everyone has a right to express themselves on fb - do what politicians and media people do.
Lest it sound by now that I am preaching, trust me - I have also been there and felt that, and it can feel very bad. However....
Welcome to academics (at least the teaching part of it) - it does not pay much, but you do get the limelight / stage for a good hour or three every day :) - and while some students may like you, some don't (there may not even be a reason to it!) and the latter often do hurt by their astringent comments, written on fb with the bravado that accompanies all of us when we are young.
Remember - as a teacher (though we all are human) we are supposed to show that we are more mature than our students and hence are able to shrug these sort of comments off (no matter how badly they might sting inside)
Sometimes, it can even help to just vent it to the class - tell them you read blah blah and you did not like the comment. the student may not stop his / her action (they may even write more often as they now know that you are affected by it), but you sure will feel better (which is important). When they write next time, do any of the other above suggestions.