So I'm an engineer at my org. I'd never want to work for Facebook. If I'm interviewing you then yes I will ask you why you chose to work there when by definition you had other options.
There seem to be a few assumptions you're making in the post above about someone you've never met and only know based on their current/former employer.
I hope your candidate in this example is able to part the conference room blinds, and point out his better-than-yours car, house, or helicopter that he took to your job site. And then just drop a microphone on the way to the next interview.
:)