Google completely fumbled Google+ by doing a slow invite only launch.

The hype when it was first coming to market was intense. But then nobody could get access because they heavily restricted sign ups.

By the time it was in "open beta" (IIRC like 6-7 mos later), the hype had long died and nobody cared about it anymore.

In my recollection, what killed g+ was forcing your YouTube account to become your g+ account, with your public name attached to the trashpit YouTube comments used to be. Everybody protested using g+, but the "Google account for everything" stuck around anyways.