How does one count users? I previously used a Skype account, but now I share a Skype phone with my girlfriend and her account. I'd guess there are plenty of families with one Skype account.

with skype all they do is look at unique logins per financial quarter, so in your scenario (and I assume it is common, because I have my own family members doing the same thing) it will count as a single user

perhaps they should adjust their terminology and refer to them as 'accounts' rather than 'users'

How many registered users does FB have in total?

I went back as far as I could, and Facebook never released total registered users. It wouldn't be relevant since fan pages, product pages, apps etc. are all in the same namespace, and there would be a lot of spam and dev accounts

I remember reading a blog post where somebody tried to calculate it by enumerating user ID's, but he didn't come to a clear conclusion.

the FB key numbers are 650M active per month, 50% of that active per day. I wish other companies were as simple and transparent with their own stats PR - instead of nonsense like 'messages processed per fortnight', 'total logins', etc. etc.