Nick over at Gizmodo reported on a presentation by Facebook's head of agency relations which suggest that Facebook has more than 10 million active users, and 6.6 million of those check in daily. However, if you try and set up an ad on Facebook and target it to Australians, then the site claims you have a potential reach of some 8.2 million people. That's somewhat lower than the 10 million "active" users claimed by Facebook in that presentation.
One possible factor in the reduced number quoted by Facebook's ad site: it only counts people over the age of 18, while Facebook allows you to sign up from the age of 13. If that's the main reason, then there are almost 1.8 million teenagers signed up on Facebook, and marketers everywhere must be spitting chips at not being allowed to target them.
Whichever way you look at it, they're ridiculously large and impressive numbers -- but when even Facebook itself can't be consistent in reporting them, a grain of salt remains helpful.