Having a Facebook profile in which the privacy settings are as strict as they can be. Users cannot search you, see your display picture, send you a message, send you a friend request, view your photos or videos, or see your relationship status, even if it is with one of their friends.
The setting is so private, two users with their profiles set this way cannot even become friends without one temporarily lowering their privacy settings.
"I saw Alex tagged in some photos awhile back. I hadn't talked to her in years, and I was going to add her so we could touch base, but she's private up the ass so I can't even send her a message!"