You say that like it's a short amount of time. Three months is more than enough time for hype to build, peek, and wither, which can -- and did, I would agrue -- kill a social network.
I was just clarifying that the invite-only stage didn't last more than a year.
Slower, with a solid core community, would have been far better for G+ IMO. It opened up too fast.
Not only didn't it have a gelled community, but there were far too many UI / feature glitches and omissions. Many of which persist to this day.