Wait, you actually NEED a law to be passed explicitly prohibiting an employer to ask for login credentials for websites over in the US?
I mean, here in Portugal the mere thought of an employer asking about login credentials for any site would send most people literally rolling on the floor laughing. Sure, they could do it, but as soon as they did, they'd basically be smacked around by the employee (at the very least, verbally), the media, and the ACT (Work Conditions Authority). And God forbid they'd actually refuse employment or fire someone because they didn't fork the info over, the legal mess would take years to clear up.
They can, however, ask you to reveal if you have an account, and how to access public posts. What you do on your free time is your own thing, but if it damages the company's image, it can still be used against you, but ONLY if it's public knowledge.
That's the way Portugal has handled privacy on the work for a LONG time. Even most medical conditions are NOT to be known by the employer (he can ask you to subject to certain tests, but the doctor can only say if you're fit for duty or not, for the most part), let alone who/what/where you do in your private time.
Seriously, does the word "privacy" have to be defined each and every single day, several times a day, these days? You would hope not, but clearly it does, if you actually need the Government to remind you of it in the form of a law...