I tend to keep numbers in my head a lot, so if I see <X> friends today that's okay. But tomorrow or later tonight that number changes to <X-1> or <X+2>. This also happens a lot 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?
I believe that the only things that affect your friend count are: if someone friends/unfriends you, someone activates/deactivates their account, or if someone deletes their account permanently. I don't think there are any privacy settings that would prevent you from seeing which friends you have.
Deleting a profile:
You'll notice that the query delete facebook has been constantly on the rise, but a few months ago, there was a sharp fluctuation; this was probably because of Quit Facebook Day where people permanently deleted their Facebook accounts. It could just be that people are fed up with the privacy issues and are deleting their profiles.
Deactivating a profile:
I know that I've had friends that find themselves on Facebook way too much and deactivate their account in a attempt to be more productive. This usually fails though, as they reactivate their accounts shortly after.
Unfriending:
As far as unfriending you, you seem like a pretty decent guy, so that's probably not the reason the numbers are changing : ) However, you could try to use one of these methods to see which friends are being removed from the list. Once you find someone that is no longer on your friend list, you can try to access their profile. If you can access it, that means they unfriended you. If you can't, they either unfriended and blocked you, deactivated their account, or deleted their account.
I have very few friends on Facebook and can easily notice an addition or deletion though I have these fluctuations as well without anyone actually being added or removed.
To scale on the level of Facebook, I'm sure they cache a lot and have multiple locations that aren't always in sync. Maybe just a symptom of the BASE (basically available, soft state, eventually consistent) model.
