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).