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If you have an STD you probably won't add some random person you had sex with on Facebook. If they add you and accept you? No STD. If they add you and ignore you? They got an STD and now you might too.
Bob added me and accepted my friend request. No STDs. I guess.
Sally added me but didn't accept. She must have herpes and now I'm worried.
Joe tried to add me but I ignored him. He probably has syphilis and I'm now infected.