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When two people are fighting and neither wants to look like a loser they sneak onto each other's Facebook to find some stupid post or photo to use as proof. It's like digging through someone's trash but with more drama and less smell.
I saw your ex's face in a photo with your new girlfriend. That's evidence enough for me.
He posted 'I miss my old life' right after I broke up with him. Classic.
She changed her profile picture to a cat. That's not just a profile picture, that's a message.