This is one of the most tricky areas in deciding what is reasonable. Obviously there are advantages to being able to share personal data about other people for your own benefit. On the other hand, every time you do that, you are potentially giving someone personal data without the subject of that data's consent.

I don't think we fully understand the implications of modern technologies in this area yet. However, I suspect we'll be learning some lessons the hard way over the next few years, as the correlation and processing of that data starts to catch up with the volume of data that's been collected.

It's also possible that some of the people giving up that data about other people, or more likely the businesses that encourage individuals to do so, are going to come under a lot of scrutiny even in terms of compliance with existing law and regulations. For example, if you install a social network's app on your phone, and that app uploads your contact list complete with names and phone numbers to their database, then both you and the social network have obviously just compromised the privacy of everyone on your contacts list. That much is black and white, but if the social network concerned then uses that data for any purpose other than providing whatever services you are explicitly requesting in terms of the contact list you already had, then from a data protection point of view it becomes a lot more just black. I'm a little surprised that data protection regulators, particularly in Europe, have taken such a hands-off approach to this issue for as long as they have already.