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 Facebook.com each month. That’s not shabby — but it’s a far cry from the MAU claims of 850 million.
Update, January 28, 2021: I stopped tracking Facebook's claimed user count, but it kept growing, and the larger numbers were always accepted and passed along uncritically by journalists and bloggers. As of today the claimed number is 2.6 billion "regular users." I wonder how many it really is. The S-1 SEC filing that Ritholz checked 9 years ago is a one-time statement made at the time of a company's initial public offering. Further research is needed to see if a tech company's regular 10-K filings must also accurately state user counts, so the 2.6 billion claim can be fact checked.