Good point. I like to talk trash about Facebook's PR but one thing they consistently do correctly is report total user numbers only when they are confirmed active users.
That said, I still give Facebook credit for doing it.
The definition of active is so obtuse and varies wildly from company to company that it doesnt even matter :).
From Facebook's Statistics Site
More than 500 million active users
50% of our active users log on to Facebook in any given day