Sometimes social network sites are the most honest form of references you can find on an prospective candidate. And while some people express preferences or display aspects of their lives that put them in a protected class, one we're legal bound not to ask about, it is information that they choose to display in association with the name they use to seek employment. Personally, I try to ignore that stuff while I look for aspects of their life that may relate to their capability as an employee. If you are concerned that you might be denied employment because you <whatever>, use an alias.
On the flip side, some candidates reveal things that make it very easy to weed them from the process for reasons that, legal or not, are in the best interest of the company and staff. The most recent in our case was a candidate that wrote us a particularly angry letter about our interview process. A quick google revealed him to be a stalker who kept a record of threats he made and threats he received through chronicle of his life. We also found a separate site devoted to his lawsuit against a former employer over some other stalking/harassment type issue. Rather than apologize and try to correct our process, we bid him farewell.
Should we avoid learning all we can that is relevant to the job about someone we might consider hiring? Google provides levels of information previously only available through the use of a private eye and with the good comes the bad and unnecessary. So we have to ignore religion, age, race, gender, preferences, et cetera. But hiring managers have been doing that for years, this information often comes up or can be inferred during an interview.
This policy seems like a Luddite decision. It would probably be better for HR to do the research and then filter out the protected information so the hiring manager doesn't get tainted. Then the hiring can be done irrespective of protected class status and yet with full awareness of the relevant data.