That is exciting; however, as someone pointed out, it's a bit misleading because Facebook was only open to college students for a few years before it opened to the public. Twitter is a good comparison, though, because it was always open to the public-- although even Twitter came years after Facebook. I think we should consider how much the previous social networks primed the public for adoption of current social networks. You might give credit to Facebook and Twitter (and Flickr et al.) for paving the way for adoption of Google+.
Personally, though, I wonder who all these Plussians are and why I don't know them! I invited all my Facebook Friends to join Google+ and only a few did. The vast majority of my contacts on Google+ are contacts from Flickr-- early adopters who rushed through the Google+ gate the moment it swung open. The second largest number are friends and acquaintances I follow on Twitter, most of whom I met at Tweetups.
Just sharing my individual experience. The growth of Google+ is still exciting to me, and I feel very comfortable with Google+ right from the start. It's clean, intuitive (mostly), and well, pretty!