Google's spent four years frog-marching its users into G+, its faltering social network, even tying company-wide bonuses to G+ performance, thus ensuring that all of Google's offerings did everything they could to cram us into G+ -- but it hasn't worked.
Google won't dish detailed stats on G+ adoption, but Dredmorbius used Google's own sitemap, published to aid crawlers in spidering its service, to analyze a theoretically representative sample of G+ profiles and to show that the service has extremely poor uptake and adoption.
G+ was full of missteps from the beginning, starting with its disastrous "Real Names" policy, which alienated the people who were already unsatisfied with Facebook's own version of "Real Names." The merger of Gmail with G+ meant that those of us who maintained a token Gmail account solely to activate our Android devices suddenly found those devices inundated with stupid notifiers about a social network we'd never consciously joined or used, flooding into our theoretically unlisted mailboxes.
But ultimately, G+ is an example of Google's long history of insecurity. In the old days, the easiest way to get Google to do something stupid was to have Yahoo do it first (see, China). Now you can get the company to throw itself off a bridge by telling it that Facebook's really betting heavy on bridge-jumping.
Summary of findings:
There are about 2.2 billion G+ profiles total.
Of these, about 9% have any publicly-posted content.
Of those, about 37% have as their most recent activity a YouTube comment, another 8% profile photo changes (45% of all "active" profiles).
Only 6% of profiles which have ever been publicly active have any post activity in 2015 (18 days so far).
Only around half of those, 3% of active profiles, are not YouTube comments.
That is, 0.3% of all G+ profiles, about 6.6 million users, have made public G+ post in 2015. That's ~367,000 users posting daily if each posts only once (the actual post frequency will vary somewhat).
This doesn't include non-public posts or comments, or lurkers, but it's a pretty clear indication of the level of publicly visible activity on G+.