When one member of a facebook relationship decides to alter their relationship status to reflect a discontinuation ("single") or troubled/sketchy state ("it's complicated" or "in an open relationship") of said relationship unbeknownst to the other party or parties of that relationship.
Candice wrote on Greg's wall "never facebreakup with me again b/c it made me break out. love you!" when he cancelled their relationship on his facebook last weekend. Now they're "engaged".
The act of changing your facebook status from "in a relationship with" to "single" and thereby notifying your known world that you are now officially single - since it is only official when facebook says it is.
(Not to be confused with face-to-face break up - which I have found possibly lethal in my past)
It's official now. Dan and Michelle are no longer dating since they both did the facebreakup. :-(