The whole premise of G+ is that it's built around private sharing with your circles. There's a lot of public sharing, sure -- but it's INTENDED to be private. That was the whole selling point for why people chose to use it over Facebook. My G+ feed is constantly being updated in a very lively manner with both public and limited posts by a variety of people.
The study is based on a flawed premise. They should find some other metric aside from "public posts" for determining how engaged the userbase is.