Facebook user agreements are "clickwrap agreements", and are legally enforceable contracts
I'm no lawyer, but it seems obvious to me that companies that ask for a job applicant's Facebook (or Twitter, or any other social network) password are asking for legal trouble:
Facebook user agreements are what are known as "clickwrap agreements", and according to most sources are legally enforceable contracts, as long as they do not violate other laws or contracts. Enticing someone to break this or any other legal contract is a violation of the law. And I don't believe the willingness of the applicant to surrender their password is relevant.
http://www.wisegeek.com/what-is-tortious-interference.htm
Tortious interference occurs when a person damages another persons contractual relationships or other business relationships on purpose. Under such circumstances, the victim can file a claim against the person who adversely affects a business contract or the enjoyment of the property right. These claims are often pursued in actions between competing businesses. Typical actions giving rise to liability include inducing customers to breach contracts, enticing employees to leave, and making false statements about the competing business.
Some legal expert, please correct me if I'm wrong.