G+ isn't the social network. G+ is their "social layer" on top of their existing services. When you take a picture on your Android phone, you have likely become an active G+ user as it uploads the picture to your G+ account. And this is likely extended to all sorts of other products that Google offers, such as Google Pages, Maps, etc. So you can't compare Google+ numbers to Facebook. Many of the user interactions with G+ are so soft that are barely "active".