Cols likes a smoke and has tried many different drugs. He has three piercings and is in the process of tattooing his arm. He earns between $75,000 and $100,000 a year and doesn't see his dad.

I know all about Cols even though I have never met him and probably never shall. Five years ago only a close friend of his would have known such personal details about him. Yet thanks to his profile on the social networking website MySpace, I even know the first thing he thinks about in the morning.

There's nothing unusual about this. Millions of people share some of their most personal details with total strangers on the internet via sites such as MySpace, Friendster and Facebook. The dangers this can pose to children are well publicised, but it also has powerful if less well known implications for us all. The sheer volume of personal information ...