http://www.ccs.neu.edu/home/amislove/publications/Inferring-WSDM.pdf

Friends share social attribute, e.g., school. Thus, even if you hide your profile, it could be predicted from your friends’  profile.

This is so true… When I was looking for people to follow on twitter, I started from some engineers I know of. After some time, I was able to follow many people working for the same company with my friends.