Your point, as I am understanding it is something along the lines of, "as a responsible employer, it's incumbent upon me to gather as many data points about a candidate as possible; sure some of them may not correlate strongly with whether a candidate will be successful or not, but every bit helps me get a full picture." It's an understandable position to some extent, but here's the reason I think it rubs people (myself included) the wrong way: people discriminate between their public (read: work) lives and their private lives. Some of your data points are public, some are not. For example: the reason that no one has an issue with you calling my boss to discuss my history if I'm applying to you for a job is that you're discussing the public face I present at work each day: "Is this guy reliable? Does he make deadlines? Can he talk to clients or customers?" In other words all things that directly bear on how well I do my job. GPA is—to some extent simliar—it's the most useful metric in dealing with candidates who don't have work experience, and the underlying question is the same: this candidate's "job" for the last four years was getting her CS degree, so how good was she at it?

Social network stalking is a bit of a red herring. If a person has bad enough judgement that they leave a public record of (crazy|nasty|racist|sexist|etc.) sentiments, that's one thing. But would you the employer ask me the job candidate to friend you on Facebook because I've restricted my posts and photos to being viewed by friends only, and you want to rifle thorough them to find evidence anything objectionable? I doubt it. To me this is the same thing as asking for my credit score: why should I reveal my personal life to you? [1] At the risk of drawing too broad a point, this kind of thing is part of a continuing and subtle erosion of employees' right to their own private lives. Companies that ask me to work indefinite 60 hour weeks as a salaried employee are of a piece with those who want to know my credit score: they are companies that are asking for more of my life than I feel they have a right to, and I will pass them by. You may be OK with losing candidates who don't want to provide you their personal data, but if a credit score really is only a secondary (at best) data point, why risk losing an otherwise-superb employee over it?

[1] To those who say "a credit score isn't all _that_ personal, I think of two examples right off the top of my head. I decline to hire applicant A because his credit score was low. As is his right by law under FCRA, he asks to know if that was why we declined to hire, at which point he may then say "My credit score is poor because my wife left last year and I'm putting 3 kids through school," or "I'm HIV+ and my last job had terrible health insurance." There are lots of very personal reasons a person my have a bad credit score, and by basing a hiring decision on that, I'm forcing them to revel them to me if they want the job; that personally makes me very queasy.