(I do concur that Circles allow this partitioning to be modeled much more accurately than most other social networking systems, but the partitioning before was entirely organic and natural, not constructed. I will wait and see how well they implement the integration before conclusively declaring Reader to have jumped the shark, but honestly the social features ("just enough social to get by", which is about as much social as I can put up with) were the only things keeping me on Reader up until this point.