Facebook recently filed their quarterly "10-Q" official report, and there are a few interesting details. CNet has picked up on the fact that a lot of users accounts are various kind of fake and duplicate accounts. Although this should come as no surprise, it is interesting to see the numbers straight from the horse's mouth (even if that probably makes them somewhat biased).
The fake accounts are categorized in three types: 4.8% duplicates, 2.4% "misclassified", and 1.5% "undesirable", for a total of 8.7%. Duplicate accounts are most likely to be under-estimated, as one would assume this is a very wide-spread user-pattern amongst young people who want a bit of privacy from their parents. The misclassified part is interesting as includes pets, toy animals, and possibly some businesses. Finally, the undesirable are the hundreds of John Rambos, Britney Spears and less famous yet non-authorized use of somebody else's name.
Using the number of 955 million monthly active users, and subtracting the fake, we're left with around 859 - 872 million (8.7% - 10%) semi-active users. Looking at daily active users, the Facebook report states that there are 552 million, but also that many mobile users leave the Facebook application running, without actively using it. They estimate that this is the case for some 5% of daily active users. So, assuming the same proportion of daily active fakes, we're left with around 470 million users (minus 15%). Or, in other words, around half of their bragging number is in fact active.
Facebook is still big, however they have a vested interest in over-representing their user count. Their latest report shows that their top number should always be taken with a grain of salt, and that around half of what they count as active users is a much more realistic number.