What circles do you think you have been added to?
tldr:
Google+ introduces a new type of privacy problems which I can only describe as the inverted personal privacy dilemma. It is no longer an issue of “What I choose to share with Google and then what Google shows to others”, the issue is now: “What friends/complete strangers choose to share about me to Google, and my inability to do anything about it”. The responsibility of maintaining my personal privacy has been partially removed from my hands, and placed in the hands of the people of internet.
Thought experiment:
Put 100 privacy aware/concerned people into a room. What would you suppose is the best way to create a global interest graph/taxonomy for this group of individuals? Algorithmically? Manually? First, ask them to fill out very basic personal information about themselves. We will probably have reasonable success. Next we ask them to fill in very specific personal information/categorizations about themselves. How do they react? Probably not as successful. Now ask the same group of people to write down and categorize the people around them, but remind them that their categorizations will remain secret. Each individual’s privacy has been maintained, and collectively we have created a very rich interest graph/taxonomy which is possibly far more accurate than any automated solution.
Ian Chan's Blog 