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1.
(N) Facebook applications, such as zombies, which really does nothing but makes you "bite" other people to level your self up. In other words your "zombie" gains 5 experience per bite so all of your friends are bitten, and all the ones who chose to be infected infect their friends and so on.
"Damn Dani, i've gotten 6 new Facebook STD's today, zombies, werewolves, slayers, vampires, pirates vs. ninjas, and super poke!"

"Hey isaac's really getting into facebook... he sent me about 10 differnt facebook std's... He's slowly loosing his life because he spends an hour a day clicking "bite 20""
by Eric Super January 05, 2008
 
2.
A virus spread from friend to friend contact on Facebook.
Like spreading a real disease any number of Facebook viruses spread around from post to post is is referred to as Facebook-STD.
by 19wisdom76 November 14, 2011