I've interviewed my share of people. In fact, we just finished hiring for all our open positions and I interviewed most of the candidates. But not once did it even occur to me to check out someone on Facebook or LinkedIn or even google them. What people are doing on their own time simply isn't relevant to the job and I think they're smart enough that if they have something useful and relevant online (like OSS contributions/projects), they would mention it since it can only benefit them.
Am I out to lunch here? Are you guys really googling people you're going to hire as a matter of course? If so, what useful information are you getting from it?