Google Plus is a beautiful product, it was built exactly the way it should have, fully integrated with all of the other Google tools, effortless in removing all barriers from users to connect and share content, a logic approach from a logical company that does not do anything out of pure chance.
In Google’s ideal world everything is interconnected, all systems are intertwined aware of one another very much like a living organism where separate parts of the system work together with the same end goal, the problem is that the principle behind the product is based on the assumption that Humans beings value convenience and utility equally on all aspects of life, after-all it worked for search, mail, and mapping, why wouldn't it work for social?
Human Interaction is not as linear as it may seem. Context is important. Once a user goes into Facebook he/she switches off work mode and the context changes completely, users are surrounded by people they know and care about (well most of the time) in an environment that does not resemble anything that can be connected to work.
This is precisely where things turn sour for Google Plus as it is not recognizing that context affects the way users interact with a system, and so the biggest strength of the product, its close integration with all the Google ecosystem, becomes its biggest weakness. Work and leisure may be all part of who I am, it is important for me that a boundary between both parts of my life to exist.