It's an acronym for Delete your Facebook. The proper use for dyfb is when one says something that you find offensive or just flat out dumb one would respond with dyfb or delete your facebook.
It works on many levels due to the fact that facebook is such a social norm that without one you basically do no exist. It's almost like telling someone to shutup or to go die.
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).