On the other hand, in the private sector, I don't see how this is any of our business. Should employer be "allowed" to "ask" for Facebook logins? You mean, should employers be "disallowed" from asking? Of course not. And candidates should say "no" if they don't want to give the login up. Employment in the US is at will, and outside of race, nationality, religion, gender, &c, hiring is totally at the discretion of the company.
†(at least, one that doesn't require clearance)