The act of adding someone on Facebook, and not being confirmed or ignored, but not being given an answer. Thus, you may not send the person another friend request. Thereby, placing you in Friend Jail.
"Hey I tried to add that hot girl on Facebook, and she hasn't added me yet, but I can't send her another request. Maybe she doesn't get on much."
"Look at her wall dude, she's on everyday. You're in friend jail."