Maybe they want to work out the kinks before having 10 million users instantly. The privacy aspect of this will be very important, and the Buzz launch is an example of how opening up to everyone at the same time led to a large backlash.
There's a pretty wide gulf between opening up and forcing it on people. You can't build a social networking sites if it isn't open to people in your users' social networks. But you also can't build a site people trust and want to use by forcing it on them.
You can get away with a lot more when you're a college student project than when you're a multi-billion-$ corporation.
It does not seem like the Facemash data was reused for thefacebook.
Page 6, specifically: http://www.wired.com/epicenter/2011/06/inside-google-plus-so...
Now, if they make it so that invited users can invite a good amount of other users (say, 100), then I think they _may_ be able to salvage this. If not, I'm not predicting a good outcome.
I actually enjoyed the idea/concept of Wave, but I never had any reason to login and actually use it since I only had one other person I knew with an account. The same will happen to Google+.
If this product gets any traction, it seems like a near certainty that Facebook will launch a "Circles" UI for their groups feature, very quickly, no?
I understand why Google is taking it slow, but if the idea sounds like old news by the time Google lets me invite people, I suspect they're going to seriously compromise their odds of user adoption.