If a previous facebook user hasn't logged out of their profile, the next person may come along and edit details about this person.
Aron; "Peter, why does your profile say "Peter is bumming"?"
Peter; "Because Ian face jacked my profile"
When someone leaves their Facebook account logged in or shares their login details with someone else at which point someone makes posts to said person's account without their knowledge.
"Bill just got facejacked. That guy gave me his login credentials for Facebook so I wrote on his wall and updated his status."
When you log on to Face Book and see that people have been carrying on conversations on "your page" without you. People that don't even know each other just sharing the witty reparte and now it takes up your whole page!
"I went to my home page on Face Book only to find my page had been "face-jacked" by my friends about some comment I made, that now has nothing to do with my original thought!"
To have your Facebook commandeered by a close friend (or foe) for their enjoyment.
Dude Kevin's status says he loves dick" "Yeah, he forgot to log off and got Facejacked
When a "friend" posts something on your facebook wall that is publicly visible and is a blatant negative to you and your friends.
I was on facebook yesterday and was facejacked by the lame band promoting the show.
When someone adds themselves as a friend on your Facebook account without your consent while they just happen to be over and just happen to be surfing on your computer.
Andy dropped by on my birthday and while he was surfing on my MacBook he Face-jacked me.