Facebook has 650M active users. They always measure and cite active users, to their credit they do not fudge around with the numbers in their PR
Skype is 170M active (quarterly, Facebook is monthly), and 680M total registered.
A lot more people have given Skype their payment details than have Facebook - which results in Skype monetizing a lot better (but still not awesome monetization - yet, but it is improving).
Edit: Facebook has never published total registered, see
http://www.facebook.com/press/info.php?statistics
It is difficult to apples v apples with skype + facebook since their methodologies vary, but it seems that the fb active user base is ~3x Skype