what algorithm Is used to decide the popularity of a card?  
I have been a bit surprised at some of the results and being an ex software engineer of a pedantic frame of mind would be very interested to know.  I can't believe one of my cards that only sold about 40 copies was one of your most popular  Christmas cards.  There must be some time factor taken into account as well.  I guess my card was available for too short a time before Christmas to do better sales.