If you have been reassured by the claim that intelligence agencies such as the US National Security Agency are 'only' collecting metadata from millions of mobile phone and internet users (for instance, details of whom a user is emailing and when, but not the content of those emails), then this ought to make you think again:

Of course the data matter too, and just to compound your concerns about privacy, here's a new way that one form of data — high res photos — can be exploited. Reflections from a person's eyeballs can be used to identify figures behind the camera:

The pupil* of the eye in a photograph of a face can be mined for hidden information, such as reflected faces of the photographer and bystanders, according to research led by Dr. Rob Jenkins, of the Department of Psychology at the University of York and published in PLOS ONE (open access)...By zooming in on high-resolution passport-style photographs, Jenkins and co-researcher Christie Kerr of the School of Psychology, University of Glasgow were able to recover bystander images that could be identified accurately by observers, despite their low resolution.