I'm late to the party (haven't been active on Google Plus in a bit), but thought I'd chime in. My opinion is what's online is fair game. If you're interviewing someone and you have a mutual friend with them, you'll probably ask that person what they think of the candidate. Obviously the idea is that anything you're looking for is work-related. While it's difficult for anyone to set their biases aside, I've had candidates who I really loved regardless of the fact that I found elements of their online profiles distasteful. I realize that their Twitter account, for example, is personal, so while I may not like what they say or believe, that's not really what I'm looking for if I see their account. I'm looking for anything that will help me make a hiring decision.
Besides, I think it helps ensure that the person you talk to in the interview isn't just doing a great job of acting for an hour. But I suppose HR professionals have an entirely different take - or perhaps a huge debate - on this.