I think part of it might have to do with the fact that a majority of people joined it for the novelty, realized what it was for, and didn't want to put the effort in to contribute anything meaningful. A good number of people in my circles seem to have just joined and said "Yay google+" and not posted a single interesting thing.
100% of all articles claiming 83% of Google+ users are inactive don't even bother to define what inactive means, let alone divulge how they managed to calculate the number.
Even when you follow the blogspam trail past gigaom to the article they linked, you still don't get a definition for inactive.
If you keep following to http://findpeopleonplus.com, the original data source, you can click to see the list of people who are "inactive". As near as I can tell from playing with the site and comparing inactive filter to active filter, inactive means the person hasn't made a post to their public non-circle feed... if that's the case it is a fairly poor way to measure this as I have a fair number of people in my circles who are quite active on my feeds but haven't posted anything marked fully 'public' yet.
