Why do the number of addresses fall every now and then? I would assume the graph should have gone up all the time, no? When people lose interest in bitcoin, I would have thought that they would just uninstall their wallet apps or something. Are they deactivating their addresses as well?
Maybe the account was unused or inactive. It might possible the owner of wallets break the block chain rules and in return there account got removed. I Still remember broke the rules on Facebook and they permanently disable my account without giving any warning.