Just in case some unsuspecting non-technical person happens to come along, maybe researching IPO stats on the company, there is a big difference between Users and actual unique Users.
For example there are corporations that may have a different user account for each of their individual brands. If there was a way of separating commercial users form private individuals we'd see a different story.
In fact I just updated a band's website for them, besides their individual user accounts for their own private activities they have a collective one for their band. That's not a unique user.
Unique users is where the real numbers are and we'll never know that.