I tend to keep numbers in my head a lot so if I see <X> number of friends today that okay. But tomorrow or later tonight that number changes to <X-1> or <X+2>. This also happens alot during the end/start of a semester <X-15> or <X+15>.

Are my friends just constantly changing their privacy settings or deactivating/reactivating Facebook? What is the reason for these fluctuations ?