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That person at the bar who you swear you know but you can't remember why. Later you realize you've been friends with them on Facebook for years but you never actually talked to them. You're just that creepy guy who stares at profiles like they're going to tell you a secret.
'Hey, do I know you?' 'No, but I've been friends with you since 2013.'
'You're the guy who liked my post about my cat dying.' 'No, I liked your post about your cat dying.'
'I've been friends with you since 2013 but I don't remember you.'