Well if you don't know which friends have left, then you mustn't have had much communication with them before that, so really no loss to you or them apart from the slight ego loss from seeing the number get smaller.
If you really feel the need to keep track of everyone in the future you ought to go to your friends list and copy/paste all the names to a text doc and save it. Then next time your friend number decreases you can cross reference this against the current list.
We're here for a good time, not for a long time. Be kind and do what makes you happy!
