I was approached by Google recently for an SRE position. I'm not sure where they found my contact information but I don't have accounts with any of the major geek sites - or much of an internet presence at all, really.

The recruiter was a tad vague about it but if I were to venture a guess, I'd say Google scans the high-profile OSS projects and associated mailing lists. That's about the only place they could have found me.