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)
a.k.a. facebookup
It's official now. Dan and Michelle are no longer dating since they both did the facebreakup. :-(