I have no doubt that G+'s active usage numbers are weaker than Google would like, but it seems like quite the chasmic leap to declare that nobody's using it based on that.
https://plus.google.com/104092656004159577193/posts/RhnKkfTN...
Upshot:
The average (mean) publicly active profile has 25 times the views of the mean publicly inactive profile.
34% of non-public profiles show no views at all.
The median views is 693 -- two views per day if the profile's been active for only a year, less if longer.
Follower counts are far lower for publicly non-active profiles. To the 35%ile they have one follower -- for publicly active, that's the 20%ile (overall it's interesting how small follower counts are per profile).
Mean followers for inactives is 6.4, for actives, ten times that -- 66.4.
Yes, that's a weakness of the study (I'm the numbers guy), but it's also what's available. It also allows for direct comparisons with other platforms offering public posting. And in the face of a dearth of quantified metrics from Google, offers a data point.