It's just a matter of time before some disgruntled Facebook employee with insider info blows a whistle on the real subs and their real "growth". I'm guessing the number is about half of what reported and the growth is slowing down.
What about the Yahoo message boards when you eliminate the political posts, multiple aliases commenting on their own posts, and, of course, the serial message board scammers?
What % of visitors do you lose? 50%? 75%? Pick a number.
Same thing. Beware of what constitutes a "user" under different contexts. There is a real disconnect between Facebook official numbers and advertisers' expectations.