I'd add that to the first point that (and sort of answers the question in the 2nd) that the goal is to use this data to make associations and inferences to better get a picture of who the user is and what their experiences are.
An emphasis on a lack of control as to what data is collected should be emphasized. Take the FB shadow accounts for users that don't even sign up. Much of your data is provided freely (mostly unwittingly) by people you associate with or simply are proximal too.
Lazy Sunday