Anybody heard of Facebook? Facebook.com is a place whereschool and university students go to kill time and - in the digital equivalent of a hello - "poke" their friends, just for fun. It must be a great deal of fun because, according to a recent survey, Facebook has become the favourite online hang-out of young American men and women between the ages of 17 and 25. If you are over 30 and use it, you are probably either a predatory paedophile or a potential investor.

The second coming of the worldwide web is taking its inspiration from a clutch of so-called "social networking sites" just like Facebook. In the course of the past decade, many of us - especially teenagers and young adults - have quit staring at the box in the corner of the room and moved to the spare room to stare at each other instead. We do so via a matrix of websites, all peopled from the ground up, such as the self-broadcaster YouTube, the vast calling-card emporium MySpace and the virtual universe Second Life. To technology geeks all this is known as online social networking, or web 2.0. For millions of young people, it is the only culture and the only kind of community worth having. Its avenues have become huge pleasure parks through which almost every facet of human experience can be funnelled.