I've had that happen before, but it was just one and early on so I knew who it was. By the next day they were back as my friend and the number was back up. Weird!
Do you follow many celebrities? Many celebrities have closed personal pages or unfriended people they don't know in real life. If they have a fan page as opposed to a personal page they are pushing people over to those.
I've had this happen and several friends have as well---in our cases for some unknown reason people were removed somehow by Facebook. Friends asked me why they were removed off my Facebook and I hadn't taken them off
This happened to another friend last week, her friend count went down by 12 mysteriously then she spent awhile trying to find out who was missing.