http://en.wikipedia.org/wiki/The_Lives_of_Others
In the film you can see some of the techniques used by Wolfgang Schmidt and his colleagues.
I don't use Facebook and I host my own email, mostly because I'd like to keep my social graph private.
However, Google knows everyone I'd have added on Plus already - and offers to add them for me. How? They've all uploaded their address books, and they all use gmail.
80-90%+ of my social graph is already known to them, just because my friends have already given them the data. My lack of participation in Google Plus is insufficient.
Even if you try and secure your data and privacy, unless everyone you know does the same thing it is pointless.
...Wow. I find it hard to even fathom that.