- "not so long ago everyone thought Facebook was close to saturation"
- "now suddenly they managed to increase the number of users"
Maybe "everyone" was wrong? In any case, faking such numbers on a large scale would require a decent-sized conspiracy among at least a couple of department heads. Also, it would be quite illegal. And, being a digital thing, very easy for someone to prove.
However, it's possible that the way they characterize numbers has changed, but those details would presumably be in the official filings and financial reports. For example, in recent years, the newspapers audit circulation bureau has changed readership numbers to include categories of web users, not just print circulation. And even before the web, the numbers included an estimated number of readers who read non-bought copies of a paper (i.e., papers left at the coffee shop and passed around).
So there could be different standards of counting...in fact, that's a much easier, more legal way of doing things before you start maliciously faking stats.