If an employer asks for my Facebook password (or equivalent, as I don't use Facebook), I plan to tell them that I don't appreciate being asked to give out my information. I don't want to work for a company where I need to put spin on an argument to have them not violate my privacy.
It's also about law enforcement jobs where they might hold it against you if you refuse. You have a right to refuse, but then they can just deny you the job because you "obviously" have something to hide.