His response was basically that they are "trusted partner" (quotes because I can't remember the exact term, but that sounds right) of Facebook and so they get all the data somehow. Maybe they pay for it? I am not sure how it would work because it couldn't be anonymized for their service.
Anyway, he basically said that, when you combine the Facebook location based posts and FourSquare checkins (and probably location anchored Tweets too as well as others I am missing), there are so many millions of these happening everyday that some of those people are bound to be qualified for the position a company is hiring for.
...Which brings us back to problematic assumptions. Garbage in, garbage out.