see Google+ Usage
I read an article stating the above link showed 186 million. When I clicked it two days later, I get 189 million, one million more per day. If the lights dim, we’ll know why.
Imagine trying to keep ahead of growth like that… Fortunately for Google they use FLOSS and the incremental costs do not include licensing fees going to M$ per server. There’s still hardware, storage, RAM and bandwidth, though but it’s probably costing Google several times less per client than using that other OS.
At this rate, M$, FaceBook and others will not be “all-in” for the cloud with no competition.