Analytics and visualization blogger Kevin Anderson studied data compiled by Edward Morbius, who says that just 9% of Google+'s 2.2 billion users actively post public content.
And of that "just 9%"... how much was activity on other Google properties where you have to jump through hoops NOT to have it cross-posted to Google+?
I know they mentioned YouTube comments, but this was also an issue with YouTube videos. If you posted one on your channel, you had to take steps for it not to be crossposted to your Google+ page. And, of course, most people who post on YouTube were semi-forced to create a Google+ page simply to remove some artificial constraints Google added with the goal of attempting to force Google+ adoption.