Sneaky creatures that lurk in the shadows and gather intelligence/date from facebook pages of loved ones, friends, enemies, and coworkers. They never comment on status or even like a photo from time to time. But they are ever so present and mind-devouring all updates on your page. In the future they may use these status updates against you in a random conversation in "real life". Be careful as a facebook phantom wears many masked and cannot be stereotyped. It could be your friend, foe, or even a stranger.

to tell someone to calm down and relax.

A person who uses facebook and uses the "go offline" function. You can see them writing on other users walls, taking quizzes, changing their status etc. But when you want to chat with them, they appear to be offline.