Face-out is a combination of Facebook and fake-out. It is used to describe someone who likes a person's Facebook status, waits for acknowledgement for liking it or long enough for the person to get a notification for it, and then purposely unlikes it. Usually, public shame and ridicule towards the recipient follows.
Sarah (posts Facebook status): I just left my house and I already miss my lil kitty. I luv you Otis!
John likes her status.
Sarah (commenting on her own status): Hey John! haha isnt he just like the cutest?
John immediately unlikes her status.
John (responding to her comment on status): Face-out! Seriously, nobody cares.
Other common forms: faced-out as in, "you jus got faced-out!"