OH MY! I just thought of a game function that would explain away characters that disappear for an extended amount of time! 

Picture this: 

Any time a user goes silent for over 4 or 5 days, the character falls into a coma and is transported to a local hospital or medicine man. Whenever their schedule frees up enough in the real world, they can wake from this coma and continue posting. This will be explained in the lore as a mysterious illness that began 1000 years ago, hence why side stories all take place before that time period. Also, each person is given a countdown. If they succumb to this illness a certain amount of times, their character will be considered dead and they will have to create a new character. 

EDIT: I'm actually expanding on this idea. This is so fun!!