There is a Facebook application that is surprisingly difficult to find called the "Friend Tracker." Once you set it up, it will tell you how many friends you have added, and how many friends have "unfriended" you, in the days since you last refreshed it. If you want it, get it here.
Any thoughts on why this Facebook application isn't more popular? Is there a more popular version that i'm not aware of? Or do people simply not want to know who has unfriended them?