Are they including Whats App users in that number? Because that would help explain it.
I don't think the numbers include instagram either, it hasn't been monetized and therefore irrelevant to financials.
Edit: Confirmed, doesn't include instagram users. In the earnings slides http://files.shareholder.com/downloads/AMDA-NJ5DZ/3349527922...
> The number of DAUs, mobile DAUs, MAUs, and mobile MAUs do not include Instagram users unless they would otherwise qualify as such users, respectively, based on their other activities on Facebook.