When you have so many email, Facebook, etc threads going that if someone doesn't include your previous email/FB message in their reply you can't remember what the subject was about.
Charles to Camilla on 01 April 2008:
"Oh Camilla, I do love those English thighs of yours!"
Camilla to Charles on 27th August 2008 (not including original message):
"Oh Charles, they love you too!"
Charles to Camilla on 14th November 2008:
"Huh? Wot R U talking about - I have Threadnesia"
NOTE: They don't FB much...