A digital booty-call. Someone you add to your friendlist just because you want to make sweet passionate love to him/her.
boy: Errm, do I know you?
girl: Nope, but you're pretty, so I added you.
boy: And how does adding me over pretty benefit you?
girl: Oh this is no reguler add, this is a booty-add.
girl: Who is nastyboy43?
girlfriend: Oh that's my latest booty-add, think he's hot?
girl: Hella! I'm booty-adding him too.
girlfriend: Have him, can I borrow that sparkly top?
boy: I didn't know you knew Lucille.
friend: Who? *takes a look at friendlist* Oh her, total booty-add.
boy: Dude, that's my girlfriend.
friend: High five!
boy: High five!