An act in which one looks at a friend's/stranger's facebook profile, pictures, and recent activity. Everyone denies their involvement in this act (when in reality most people do it when bored), then in turn accuse someone else of doing it later.
A person that does this often is called a Facebook Creeper.
Jon: "Hey, did you hear Jack broke up with Jill?
Paul: "Uh, how'd you know that? Were you facebook creeping?"
Jon: "No, of course I wasn't!"
A few hours later...
Brian: "Hey, did you hear Jack broke up with Jill?"
Jon: "Dude, stop facebook creeping."