Right now all I need is to google your name to find out a LOT about you.
If I have your email address, then I know all the social networks you're in, what you've recently tweeted and where you work.
So the question becomes: when will I just be able to take a picture of a stranger on a train and instantly know their name, social networks and workplace?
Facial recognition is coming a long way...hell, ads in stores are looking back at you and determining if you're a man or a woman and tailor (pun intended) ads to you. License plate recognition already scans hundreds of cars as they fly by and give officers real-time data if you're a criminal or not.
When will the general population be able to do this with their friends. Can they already?