That is a bad definition of "active," particularly considering that the original selling point of G+ relative to Facebook was its privacy controls (circles). I know many people who post to G+, only a few of whom do so publicly.
Both Enge and I address both the definition of "active" and of "invisible" non-public activity.
1. We're limited to directly observable public actvity. Given the sitemaps providing a population of profiles to sample, that means looking at actual public posts. Anything else would be Making Shit Up, which I prefer not to do.
2. It's possible get some sense of non-public activity by looking at followers and views for profiles with and without public activity. Evidence is that profiles without public posts have about 4.3% of the activity of those with. That gives roughly 4% of the total G+ userbase, or about 95 million users.
Again: showing activity at any time. That about doubles the active users count in total. (112 million + 95 million). It doesn't speak to recent activity, though.
