There’s a big debate going on in Norway these days about Datalagringsdirektivet, the Norwegian implementation of the EU Data Retention Directive (2006/24/EF). Basically, the directive allows the police to have access to communication data for everyone. Who is talking to who, who sends mail to eachother, etc. etc.
Now, think about this. What the police gets here is a graph of people that are talking with each other. With a little bit of statistical analysis and the appliance of a couple graph clustering algorithms they can find groups of people that talk to each other regularly. Now, these groups might be called families, friends and such – but what if the clustering is of something more nefarious?