1) Posting or commentating on Facebook when under the influence of alcohol or in a highly emotional state. Upon sobering or waking you wonder if you did log onto facebook whilst under the influence. Then it dawns on you & you reap the nightmarish/highly embarrassing effects of your actions or lack of tact.
2) Having someone else posting, commenting or associating you with something on Facebook that results in the above nightmare situation.
I've had a Facemare, I got wasted last night & posted what I really think of my boss & co workers!