Facebook just removed 584 million fake accounts. Isn't that like 1/3rd of their user base?
FB ususally reports DAU/MAU (daily active users, weekly active users). That means, if you remove 500 million of not active users (not using facebook last month), these numbers are not affected. I don't think this would explain the majority of 500M number.
Most of the 584 million removed fake accounts were most likely created in the same period (and most of them were removed quickly after registration -- as said in the FB report). So they didn't decimate their user base, but additional 500M accounts that were deleted (so they shouldn't affect the user growth). This assumes vast majority of active fake accounts will be blocked.