A person who cannot think for themselves and relies totally on facebook for their interactions with the real world. To them nothing is significant if it's not on facebook, and not posting an event on FB means they have every right to ignore it.
I.E. birthdays, breakups, graduations, engagements etc...
Chris: I'd like to thank everyone for the wonderful birthday wishes yesterday. Wait, never mind no one remembered and you are all jerkfaces.
Everyone: What do you expect, you didn't post it.
Chris: Well you've known me for so many years...guess I'm old fashioned.
Everyone: OMG yeah, duh.
Chris: FB Zombies!