to 'nap' / to hold someone's facebookprofile; either by hacking it or simply when you want to check your account and your predecessor forgot to log out.
a 'facebooknap' results in two ways:
1) you're profile is now home of some random spam agency
2) you're profile is filled with random/hilarious status updates, pictures, ...
person 1: OMG, Why is there 'I <3 cum on my face' on my facebook status?
person 2: Did you log out when we were at the libary?
person 1: Oh no I forgot
person 2: classic case of facebooknapping!
person 1: oh look, someone forgot to log out on facebook.
person 2: let's mess with his account
person 1: time for a facebooknapping!
person 2: lets put 'I <3 cum on my face' in his status, it will be hilarious