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Of those 10m users there are only 5 that I know. If they had opened G+ up to all current Gmail accounts and Apps accounts they could have made a huge impact, there was enormous buzz around my college campus (where everyone had an Apps for Education account) but nobody could get in. When FB opened itself up to non .edu accounts it had swept my school in weeks because there was no barrier to entry and people wanted to try it out. Even though there were far fewer users on FB then than G+ now you had social circles joining en masse creating a foundation for a usable network. For most younger people your group of friends likely joined all around the same time, which couldn't happen with G+.

Instead of universal limits on access, Google should have kept the first iteration a truly closed and internal beta then opened it up to everyone.