Social networking sites like MySpace are among the most popular destinations on the web, attracting millions of members and millions of dollars in investment.

Are these online communities the future face of Web 2.0, as their proponents claim, or just another passing fad? What's driving their popularity and how will they develop in the future?

MySpace boasts more than 100 million members worldwide. According to market research firm Hitwise, MySpace accounts for 4.6 per cent of all visits to US web pages, more than Yahoo and even more than Google.

Facebook, popular with US college students, generates more page views than Amazon's US shop. The UK's most popular social site, Bebo, boasts 26 million members worldwide.

Big audiences attract big investment. MySpace was bought by Rupert Murdoch's News Corporation for $580m in July 2005.

Google bought video-sharing site YouTube in October for $1.65bn, while Bebo is rumoured to be in talks with media giant Viacom at a price tag close to $1bn.

No one can deny that social networking is the web phenomenon du jour, especially among the young. That is no surprise: 18-35 year olds are the most active online and many cannot remember the world before web access.

But online communities are not new. Before the web, academics and computer nerds used dial-up lines to gather on bulletin-boards. In the US, sites like Buzz-Oven, based on the Texas music scene, are as old as the web itself.

The UK's first mass-market taste of social networking was Friends Reunited, which provided the low-down on what your old school mates were up to.

But it is youth-oriented sites like MySpace and Bebo that are growing the fastest. Alicia, 16, is a member of Bebo, visiting the site sporadically. "You can put on the music you like, chat, write quizzes, whatever you want," she says.

Alicia uses Bebo to keep in touch with friends who have moved away, but also to chat with people she sees every day at school. An erstwhile pen-friend, who is now a fellow Beboer, is the only online friend she's never met.

A lot of the activity on Bebo and MySpace is about young people establishing their identity. These are 'my musical tastes', 'pictures of my friends', 'my likes and dislikes'. This is me. Or at least, the face I want to present.