Fast Company notes that the Google+ notion of adding people to various circles might not be working out - and is instead leading to "circle fatigue":
Gigaom's Mathew Ingram chalks it up to what psychologist Barry Schwartz has called the "Paradox of Choice," meaning that "too much choice actually makes it less likely [users] will take advantage of a feature." He adds, "The process of filtering hundreds or even thousands of people into groups is time-consuming and somewhat frustrating," and could cause Circles fatigue. AllThingsD strikes a similar tone, highlighting just how complicated friending is on Google Plus compared to other networks.
I'm not following that many people on Google+ yet (or even paying much attention to it, honestly), but - I hit that fatigue point on day one. I've only used two circles thus far: friend and following. I'm not sure what value having two circles even gives me. Facebook still seems more useful, and honestly, having the back end prioritize the messages based on traffic patterns would probably be a whole lot more useful still.
I suspect that Google simply won't make much of wave here (pun somewhat intended). To get traction, they have to compel people to stop using facebook and move over; the sheer force of inertia argues against that. When "everyone you know" is already on Facebook, why do you even care about another system?