When someone studies peoples posts on Facebook (Comments, pictures, status updates) to the point to where they know everything about a persons life, without ever talking to that person. A classic Face-creeper is known to know way more about a victim than the victims best friends know about him.
Wasting time you don't have on Facebook stalking people, or reading their walls.
Girl: I'm so upset that Justin has a girflriend, now I have no chance.
Guy: How on earth did you figure that out?
Girl: I was reading his wall conversations with one of his friends, you know, back and forth between their two walls so I could get the full converstaion and his friend congratulated him on having a hot girlfriend.
Guy: You were face creeping at home all alone last night?!?! Maybe you should go out...