I'm not convinced that theory is correct. There's a difference between asking a prohibited question, and acquiring the answer to a prohibited question incidental to something else.
To give an obvious example, you can't ask about race and sex, but the employer is going to find out that information at the face to face interviewer by simply observing the candidate.
Or consider criminal background checks, which are allowed in some states. A criminal background check might turn up information that makes a candidate's sexual orientation, national origin, or religion apparent.
I don't think that an employer would find themselves in legal trouble for doing a criminal background check that turned up that information, because they weren't doing the background check to acquire the prohibited information.
As a practical matter, though, even if it is legal to look at Facebook, it is a bad idea. Suppose you do not hire a candidate, and the candidate sues claiming that you didn't hire them because of their marital status. During discover, the plaintiff finds some emails between employees containing disparaging remarks about people of his status.
If you have not looked at Facebook or otherwise snooped into their private life, you will offer as part of your defense that you did not know their marital status, and so could not possibly have discriminated on that basis. Even if you do have some managers who dislike people of the plaintiff's status, they could not have acted on that dislike in this case.
If you have snooped, then you no longer have that defense. You are in the much less desirable position of having to argue that your people (who have been caught disparaging people of plaintiff's status) did not use that information, even though they did have access to it.
I can easily see this being the difference between a plaintiff win and a defendant win.
This is why background checks are usually done by third parties. A background check will inevitably find out information that you're not supposed to discriminate upon - having it done by a third party means that the people actually making the decision don't know that information. (And more important, you can prove that they didn't know it)
Even on those cases, it is not obvious. A person that looks like a man might consider himself a woman, for example. I'm currently working on the Chilean Census, and, even though we are supposed to ask for the person's sex, we can't make any judgment about it. If we enquire further than what the person says, we are in for a lot of trouble.
In England we have Sex, Sexual Preference, Race, Religion, Age, and Disability.
There's a Rehabilitation of Offenders Act which covers what you're allowed to do with people who have a criminal record.
You are not innocent until proven guilty, it's "balance of probability". And given that the focus of Facebook is person information, it's hard to argue that you weren't interested in it.
You don't just have the information, you have shown that it's the kind of information you are interested in. Unless you are looking for very specific things (in which case, why didn't you hand it off to a third party?), their lawyer will say you were interested in the general "look and feel" of the candidate. Stuff like age, marital status, religion, what their friends and family are like. All the stuff you shouldn't ask.
In an interview you can't ask how old someone is, where they were born, what religion they are, and a zillion other things, all of which are blatantly plastered all over the average person's facebook profile.
While I would certainly not put up with a company asking for access to my private online content, it's not clear that it's illegal for them to do so just because it may contain information that can't legally be used to make a hiring decision.
Facebook, as seen from a logged in user's POV, is meant to be private. I think there is definitely a line being crossed when asking to snoop through their private lives.
This is just my opinion.
It's not that simple. Facebook is not the only way a prospective employer could find out those details. For example, some jobs do thorough background checks (imagine, for a security position) that no one doubts the legality of, and you could answer many of those same questions that way too. If the prospective employer learns that info, they're simply expected to disregard it while making their hiring decision, the same as they'd be expected to if they learned that info from your PUBLIC facebook page (which again no one can doubt is legal to check).
Stealing facebook login details does expose the company to greater liability claims in this regard, but merely uncovering those protected questions is not illegal.
Instead I suggest approaching Facebook login theft as a case of tortious interference, and an invasion of privacy both for the stolen account owner and anyone on their friends list. Employment discrimination will both be hard to prove, and should be settled on a case by case basis rather than legislated out of existence.
This is why competent background checks are conducted by a security officer who produces nothing but a list of disqualifications along with the factual evidence to back them up. ("The candidate appears to engage in the unlawful use of mind altering drugs. [Facebook photo of bong use attached.]")
(IANAL, and this is not legal advice.)
The laws in Ontario are written this way to prevent exactly this sort of idiocy. That's why you don't even ask these things because it's irrelevant to the job.
If illegal drug use doesn't disqualify the President of the United States from his job why should it exclude anyone else?
Just out of curiousness - what do you mean?
A lot of bongs are re-purposed chemistry apparatus, too. Not that having chemistry apparatus in your student flat is gonna look much better to a prospective employer than a bong. :P