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A Facebook Unicorn is when you run into someone you're only friends with on Facebook. They might be the guy who sat behind you in math class or the girl who added you at 3 AM after too much alcohol and a bad decision.
Hey. You're that guy from math class. Right? I think we had a test on the same day.
Oh my God. It's you. Why are you still friends with me?
You're the one who added me at 3 AM. I still don't know why.