When you post something on facebook that is crude or potentially offensive, and then immediately delete it after you realize that all of your family members are going to see it.
Man I heard this hilarious joke about taking a dump on an old lady's chest, and I posted it on facebook, but then I remembered that I'm friends with my grandma. I totally facebalked!
(v.) fusion of the noun, facebook, and the verb, to stalk; to stalk members of the facebook community so frequently that one cannot waste the time uttering the two separate words for the unbearable thought that during the slight pause between "facebook" and "stalking," more than one person may have updated their profile.
"OMG, I was totally facebalking this afternoon and saw that so-and-so wore your dress out last night on a date with your ex-boyfriend!"