I think people are convinced that services must blow up immediately to be relevant. They forget that even Facebook took time to grow. I remember when it first opened to people outside of colleges and I was an early adopter, there were literally maybe only 30 people on Facebook I knew. I think Google+ is going to be a success and the reason for that is because of the integration with other Google services, as the platform matures people are going to find little reason to stick with Facebook or other services when G+ will have all their friends plus integration into all the Google services.
Facebook did blow up in select circles though right from the get-go. When it hit college campuses it spread like wildfire as there were no restrictions to entering and you could have all your close, daily contacts together in one place instantly. Google+ let me and 5 other friends in but prevented me from inviting others and now it's basically dead with the last post between any of us over a month old.
The overall number of users in the system isn't something important, it's how many users I know. A network with 10,000 users at Harvard will spread out much faster than one with 10m dotted around the US as it represents a contiguous userbase that is a solid foundation for social interaction.
i think you hit the button. there are a large group of people who are only aware of G+ in the periphery. But as people migrate to G+ and android phones become more prevalent, people will go with "integrated ease" it def has enough features so far to pique the layman's interest. not to mention as lil kids get on the internet do they want to be on their "parent's social site" or go to the hip new one?