And that's exactly what some of us do. But there's no way you're getting all of your facebook friends to do the same. Facebook can build a profile on you based on your friend's tracking data. Whether it'll be accurate or not is mostly a technical question - whoever looks at it will assume it's accurate. There are a lot of nefarious uses for this information and there's no useful way to opt out right now, short of not having created a facebook account. Hence the whole "mass surveillance" bit.