1. When you stumble upon the page of a person you haven't seen in years and find they've changed in shocking ways you would never expect
2. When you discover that people you know know other people you know without you knowing. Sometimes better than you'd like them to.
3. Generally when you see something surprising or unexpected about or involving people you even vaguely know, and it goes beyond the level of surprise to a state where your reality is totally rocked.
"Remember that really dorky girl everyone made fun of in school? Shes a rich supermodel now."
"Oh god, my current girlfriend and ex girlfriend are friends on facebook. They have loads of pictures together too, I think they're pretty close. She never told me they knew each other!"
"What a facebook mindfuck!"