mau-mau-ing the MAU count

On the topic of Facebook's ever-upward user creep (supposedly 400,000,000 two years ago, now supposedly 845,000,000 MAU, or monthly active users), financial blogger Barry Ritholtz counters:

What I learned from Facebook’s [S-1] filing was that they have 161 million active users who actually go to each month. That’s not shabby — but it’s a far cry from the MAU claims of 850 million.