The other week, when I prepared a presentation, I spoke to Microsoft staff here in Sweden trying to get an official number on how many active users Office 365 had these days. The only official answer I got was "around 100 million". This has changed tho:
Last month (late October) Office 365 officially passed the 120 million active commercial users mark. These are monthly active users. That's a growth with 20 million users over the period of 6 months (100 million active in April). The crazy thing is that Microsoft expect the Office 365 user base to keep on growing.
Time will tell if they are right but everything points to that direction ;)