I can anecdotally confirm this to be the case. I was in college when G+ came out, and there was a lot of interest in my friend groups; except that nobody could get in.
There was a moment when a lot of people wanted to make G+ accounts, and if they had been able to do so, the network effect of them making accounts would have spurred a number of others to do the same. But they couldn't, and by the time they could, interest had died. G+ had one really good shot, and missed.
No, G+ was annoying. You had to classify people in circles. Then nothing happened.
The reason why the hype died was that it was a bad product. That’s why Youtube made it mandatory to transform your name into a G+ account. Talk about pathetic.
Again it's an example of what working before not working in the future, though. Invite-only had worked great for GMail; it actually intensified the buzz. It failed miserably with Wave and Plus, showing that the same tactics sometimes work and sometimes flop.
Gmail was invite-only to join and get an @gmail.com account, but once you had it you could interact with any email account user, and they could interact with you. GMail isn't a walled garden. Facebook, Wave, G+, etc are. That's why they depend on rapid user growth very early on when the hype is fresh.
