Many will be fake and abandoned accounts. I had heard that the growth for users in their target base of 16-24 is stagnant and people have now moved to other platforms due to privacy concerns. Also it isn't particularly cool when your parents and grand parents are also using it.
freitasm: It is one billion active accounts, not counting fake, inactive and others.
It is all to do with how it is worded and calculated though. I wouldn't call someone who only logs in once a month a particularly active user. Maybe if they released the weekly figures, which would be a lot less, it would be more accurate. I also know people who have got fake accounts so they don't want to give their real details to facebook.You can now also setup business accounts without being a personal member, so I wonder those were included