The "trail" someone leaves behind as they comment, like, or chat to you about as they scout/stalk your Facebook page. Sometimes, if lucky, you can see the trail navigate through different albums/time periods of your posts and pictures.
John: Hey dude like this pic!
Gary: Thanks man.
5 minutes later- John liked your comment
5 minutes later: John liked your picture
Gary: WTF! Doesn't he know he's leaving a facebook trail?