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.
Let's also not forget about the offshore entities that create Facebook farms of fake individuals so they can sell their "like" services for $3.99 per thousand fans of non-existing people. There is so much abuse of this type going on right now.
I know at times I come across as totally negative but I am not. I'm just trying to do my part to help bring awareness to unsuspecting individuals who can pour some of their life-savings into investing in an IPO in which the insiders are already planning their "short-selling" strategies to time it perfectly after launch.
Just have a look at what's happening with Linked-In right now for evidence.