Google + killed itself by being invite-only at the time when there was buzz. Once the invite-only period ended, the buzz was gone.
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.
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.