How many are actual users? About 50% of my friends in Google+ have no info, posts, pictures, etc., up. They're on Google+ because they got an invite and that's it. What's the use of having that many users if half of them don't use it?
Now that they have a profile, if they are loggued in there google account, as soon as some people in their social circle starts using it, the cost of interacting will be very low (because of the g+ toolbar).
That's likewise true for facebook users, or users of any online service. There's a long tail of people who sign up then walk away. But services include it in "user" counts anyway. So it's still apples-to-apples for comparison purposes.
