The way people interact with other people can not be reduced to a finite set of values, such as "friend or not friend" or "family member, lover, friend, acquaintance, co-worker". In other words, this is not a Facebook vs. G+ dilemma.
The problem here is the fact that Facebook has, yet again, limited the way we interact with other people by imposing their own structure and rules: if you don't want a person to know you're paying attention to them, don't pay attention to them.
Your proposed solution of "don't add them as a friend" is even more restricted. Maybe it's your way of interacting with people, but I don't see how it's justified to try to impose it on everyone else.
So... you're arguing that people choose social networks based on more than the presence of a particular implementation of a single feature? Groundbreaking addition to your "all your friends are the same" theory.
Then people need to use Facebook differently. Seriously. What is hard to understand about Don't friend people you don't want to share info with?
This seems like common sense advice, like "Don't knock on the door of people you don't intend to speak to".
This completely ignores the reality of how people use Facebook, as if one could partition every personal relationship in your life into two homogeneous groups "friends" and "not friends."
Yes, I know you can have friend lists on Facebook, but this feature clearly embraces its uniformity.