A.) When, upon waking, one realizes that you went on Facebook last night blacked out - resulting in untold horrors.
B.) The actions of going on Facebook while in the middle of a alcohol-induced blackout.
A.) "Why did I send six vile Facebook messages last night to people I barely know? Oh, it must have been a Facebook Blackout..."
B) "Duuudeee...I am so going to send messages to this bitch!"
"Dude, don't do it, your in the middle of a Facebook blackout!"