It is all about how you use it. If you were to start a brand new Twitter or Facebook page and did not actively engage and only followed people that were not active, you might be saying the same thing about those sites.
G+ is mainly geared towards following your passions/interests. I, like John, cannot even keep up with all of my feeds/communities/circles/etc over here. For me, there is more activity on G+ than all my other social networks combined. That is one reason why I do not have the same kind of presence as I do here on the other ones.
I have sought out those things which I care about or are interested in. You cannot do that as easily on those other sites and as a result, I care about the content over here a lot and keep coming back. On Twitter, I do not like being constrained to 160 characters and on Facebook I do not really care about the chain letters or what people are doing right this second. I feel that G+ is a good mix of both worlds. I can still keep up with people and have actual discussions with them without worrying about constraints or censoring.