Let's say you've got a classic oversharer in your Aunt Sally. It's uncouth to ignore her entirely, as sometimes she does post things you need/want to be aware of, if for no other reason than to reduce family stress.
On Facebook, there's just a steady peppering of her stuff in all the other posts. On Google+, you can shunt her off into a "noisy" circle and deal with her posts maybe once a week.
But there-in lies the rub: any given dull Facebook session you can click on an Aunt Sally link -- they're always right in front of you. But on Google+ even if you've got 5 minutes to kill on a random link, clicking over to the "noisy" circle carries a mental obligation to not just find a link, but actually do the chore of scanning her aggregated posts to see if there was anything in there about your Uncle's knee surgery or the family reunion.
So you generally don't do it. Or, if you do, you're not concentrating on the random links, you're dealing with the chore of sorting the pile, and the random links still aren't getting clicked.
And the flip side is that when you construct circles for your own sharing, you may think you're sparing your gear-head uncle bob from all your links to computing articles, not realizing that he actually did read them from time to time on Facebook.
Though this explanation does raise the question of "was the lost traffic actually valuable?" as your killing 5 minutes on an Aunt Sally post, or Uncle Bob skimming an anti-Facebook rant are certainly going to be page-views, they're unlikely to have resulted in any sales or ad-clicks.