Earning money usually means doing lots of hard and unpleasant things. Giving your facebook login information is hardly the worst thing employer can ask you to do. Why is it so special? How do you determine what should and what shouldn't be allowed?
What if your employer wants you to steal from your family? What if your employer wants to have sex with you? You either believe labor laws should exist or you don't. If you do, I don't think the answer need to be anything more than "common sense".
