I was under the impression that the plus movement died a while ago internally at Google. They had an initiative to switch all of the user ids to plus ids and have everything revolve around your plus account.
The YouTuber's (Google's biggest social platform, and displacing FB recently in Alexa's rankings) didn't like it. The G+'ers didn't like it. And the privacy nuts (self included) really didn't like it.
It took about six months for that to play out, but in the end, Vic Gundotra was pushed out of Google, and G+'s Product Manager left to join YouTube directly.
It's been fits and starts since then. A major site redesign, which despite the standard Google UI/UX fuckups, is vastly less resource-intensive under the hood, this past December. But ... little else, and very long-wearing problems still unaddressed.
The lack of meaningful search is particularly embarrassing.
I suspect what may have done more to kill this than anything though is the political cycle. I'd joked to the former G+ chief architect that "the political cycle was hard on the social graph", and got back the cryptic response that "it was more complicated than that". This all a couple of years ago.
Then 2016-17 happened.
Given the general unpleasantness of political discussion, harassment, bots, etc., and what I suspect is soul-searching on multiple fronts as to what is really worth doing and engaging in, Google are themselves increasingly tarnished.