facebook duet is when two or more friends on facebook take turns writing verses/lyrics to a song on someone's wall.
It can be started by one person on their own wall or facebook status and can be joined by anyone who can accurately site the lyrics continuing the song along.
When someone messes it up, it is clear that person should be removed from your friend list because they don't get the game nor do they have any sense to keep their mouth shut if they don't know the lyrics properly. This could possibly fit into the category of 'Facebook Douche".
Correctly Done Example -
Jane Doe: We're leaving together, but still it's farewell. And maybe we'll come back to Earth, who can tell?
John Doe: I guess there is no one to blame. We're leaving ground. Will things ever be the same again?
Jane Doe: It's the final countdown!
John Doe: Nice facebook duet, my friend.
Jane Doe: Let's make love.
John Doe: Whoo hoo!
Incorrectly Done Example:
Jane Doe: If I leave here tomorrow, Would you still remember me?
John Doe: For I must be travelling on, now, 'cause there's too many places Ive got to see.
Jane Doe: But if I stayed here with you girl, things just couldn't be the same.
Jack Ass: And a free bird you can't chaaaaaaangee! WHOOOOHHH YEAH!
Jane Doe: I hate you so much.
John Doe: Way to cockblock me, asshole.