To scan Facebook constantly, looking at everyone's profiles, interests, photos, etc. Often done on attractive people, or young boys in girls, and bets are placed as to who will grow up sexy.
Creeping is commonly done by jealous or skeptical girlfriends, to make sure their boyfriend is not talking to other girls, or commenting on their pictures.
Creeping is a common habit which is acceptable to a certain point. You know you have a problem when you know a little something about every kid in the upcoming grade 8 class, and you are graduated..
girl 1: Omg did you know ashley and john broke up :'(
girl 2: um.. no.. you never even talked to her in your whole life whats the problem?
girl 1: well i was creeping her so much i feel like were friends.
girl 2: ah. well then. continue.