Indeed. I know lots of people who signed up for G+ to check it out, I only know two people who still actively post to it. I just checked my news feed, and it's entirely filled with posts by celebs (famous photographers, web celebs, Google folks I follow, etc), and two posts by my actual friends.
If this is at all representative, I suspect the percentage of active users to total users is pretty abysmal.
It's definitely way more barren on G+ than Facebook, but I think that part of this is because on G+, users are more likely to make posts that can only be seen by a small subset of the total number of users in their circles. On Facebook, most users still make posts visible to everyone on their friends list. I personally have a "close friends" circle of about 15 people and we typically just share content with each other; at the same time I'm seeing posts from people outside of this circle talking no one ever posts to G+.
Although G+ still has way fewer posts than Facebook, I suspect that it would need a much higher post rate than Facebook to appear as "active" to users.
I think you have a really good point about circles being smaller, a side effect of publishing only to certain circles will result in less updates seen, no matter how we try to slice it.
I find G+ posts to be generally of much higher quality, especially shared things.
I believe quality of the posts depends on the quality of the people... I had(since I deactivated both Facebook and G+ accounts of mine) nearly the same sets of friends both on Facebook and Google+ the last time I checked them, and the quality of the posts were nearly the same. Actually, most of them were posting the same things to both platforms.
If this is at all representative, I suspect the percentage of active users to total users is pretty abysmal.