Marketing to friends of your existing customers is a natural goal, right? And Google has that data for maybe half of humanity, in its gmail archive. A highly saleable commodity. But try to make Google sell you either that gmail contact data or anything equivalent.
Depends on what you mean by privacy. If a computer program collects information about you to determine what products you might be interested in, but never reveals that information to anyone but yourself (by showing you ads related to those interests), is that information still private? I would argue that it is.