When you recognize the name of someone in a conversation, but then realize that you only know them through facebook and have never actually met.
Disclaimer: Admitting this could cause you to sound like a creeper.
Person 1: Sorry we couldn't hang out, I was studying with (Insert Name) last night. Do you know him?
Person 2: Hmm, not by face, but I might know by Facebook.
Person 1: So that's a no?
Person 2: Pretty much.