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1.The time of day when your list of friends on chat exceeds from one friend online to forty friends online.

2.The time of day when everyone is logged into facebook. Usually between 4:00-5:00
Man 1: Dude, I was on facebook yesterday at 4:30, there were so many people on my facebook chat broke.

Man 2: It must have been Facebook Rush Hour.
by Phil Muhweenuh April 15, 2010