One of my old colleagues, Adam Smith, has written a very interesting post called If you know a criminal, are you therefore a criminal? bringing together threads from social networks and current informal network data aggregation tools and comparing the impact of voluntary data provision to that of data about you which will be collected on your behalf and stored on government servers when the Data Retention directive comes into force.
A very interesting read, and a nice drawing together of information from different sources. A lot of defendents of the Data Retention directive cite the fact that because the contents of messages (such as phone calls, IM messages and emails) are not stored, no-one should be worried. As far as I can see, and as Adam points out, this isn’t really the case.