IT IS the sheer pace of growth that is astounding. Piczo will be an unfamiliar name to many readers, yet since last December this networking website for teenagers has grown eightfold, increasing its number of users from 1.2m to 10.5m, with nearly 4m in Britain alone.

My Space and Bebo have become well-known brands over the past year as the social networking phenomenon has spread like wildfire across the internet. But alongside these UK market leaders, a host of rivals are springing up, and some are growing every bit as fast.

“There’s this massive thing happening,” said Jeremy Verba, Piczo’s chief executive. “10-22- year-olds are coming on to the web and claiming it for themselves. This generation is grabbing on to this medium and making it their own.”

According to Hitwise, the internet data firm, Piczo has quickly become the UK No 3 among networking sites. The San Francisco-based firm, which claims to offer a more private and controlled environment than My Space and Bebo, is most popular among teenage girls.

Piczo allows its users to upload and share their photos and videos, and to express themselves through their interests and passions. But unlike My Space, there is no search facility — making it much harder for strangers to stumble across your web page and seek to become your “friend”.

The popularity of Piczo and the others explains why the teen magazine market is in such difficulties. Smash Hits and Elle Girl are among the titles that have closed this year. Cosmo Girl, meanwhile, has seen its sales fall from 173,000 to 142,000.

However, the appeal of the websites extends well beyond the core audience of 13-25-year-olds. My Space and Bebo are at pains to point out the average age of their users is older than much of their media coverage suggests. (My Space says two-thirds of its UK users are over 25.) You don’t need to spend much time on Last.fm or Wayn.com — two British-based sites specialising, respectively, in music and travel — to see they attract many middle-aged users.

Martin Stiksel, one of the founders of Last.fm, said the firm now has 15m unique users every month, up from 3m a year ago. “It’s really grown over the past few months,” he said. “We are still growing at double-digit rates from month to month.”

Last.fm is unusual in that it offers its users software that tracks every song they play from their computers. Every day, the site’s users “report” 13m songs to Last.fm, creating a huge database of what the world’s music lovers are listening to. Every week, Last.fm creates a personal chart for every user — showing the songs and bands they have been playing most.

These charts are a goldmine of information for any music geek — and a source of considerable angst whenever Last.fm upgrades its website. The customer-support forums fill with users’ complaints such as: “You’ve missed 43 songs, including two Radiohead albums and five tracks from Nirvana’s Nevermind.”

This is the other point about social-networking sites — the degree of engagement by their community of users. Millions of people spend hours working on their profiles: designing the page, describing their interests, writing weblogs and uploading photos.

According to Hitwise, Bebo’s users spend an average of almost half an hour on the site every time they visit. My Space, which has a slightly bigger UK market share than Bebo at 32%, is almost as “sticky”, its users spending 26 minutes a visit. Face Party, the UK No 4, holds on to its members for an average of 25 minutes.

This helps answer one of the questions about networking sites: how they will make money. Verba said: “Where the audience goes, the media dollars will eventually follow.”