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When you talk to a friend on Facebook through messages, usually of the opposite sex, acting as though you're together even though you're both in relationships. Something to do for fun.
"Hey Kate who are you talking to?"
"Oh, just my Facebook Boo"
"Your who? Aren't you with Derek?"
"Yea, this is just a friend I call my Facebook Boo because we talk in messages playing around."
by Dynamite!! October 12, 2011
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