The runaway success of Facebook and other social networking sites has seen hundreds of thousands of people sign up to them every day. Even though Facebook was launched just three years ago, its creator, Mark Zuckerberg, is in line to become the latest online billionaire. He has recently turned down a bid for the business, thought to be worth more than $1bn, from the internet service provider Yahoo!

Zuckerberg is fighting a court battle in Boston against three former friends who claim he took the concept, computer code and design from a site they collaborated on as students, now called ConnectU.

What are social networking sites?

They are a way of getting in touch with old friends and keeping in touch with current ones. Facebook allows each user to set up a 'profile', listing details of their school and university career, as well as allowing them space to upload photos and video clips. Each person's profile can be viewed by people listed as 'friends'. Friends can also post comments on each other's 'wall' . . . a kind of instant email service.

MySpace and Bebo work in a similar way, although MySpace has become more known for its association with aspiring bands who upload their latest music onto their MySpace page in the hope it will be spotted by bigwigs in the music industry.

Facebook also allows users to notify friends of forthcoming events, and groups of common interest can be set up, covering everything from chicken nuggets to Adam Smith's moral philosophy.

New features seem to appear daily.

Members can now hug and high-five each other, and even set up an electronic aquarium or do some pruning in their virtual garden. A new language has stemmed from social networking sites, too . . . for example, 'poke' is an electronic greeting between members. The latest edition of the Collins dictionary included the word "me-media" to describe such internet sites.

Exactly how popular are they?

Bebo has had particular success in Ireland, claiming at least one million users.

MySpace, bought by Rupert Murdoch for $580m ( 424m) in 2005, remains the most popular in the UK, with about 6.5 million users. But Facebook and Bebo are hot on its heels, with Facebook growing 19 times faster than MySpace. Although Facebook started out as a site for US college students, it is relatively more popular with UK internet users. According to the internet market research firm NetRatings, if Bebo and Facebook continue to grow at their current rate, they will be as popular as MySpace by September.

Who is joining Facebook?

At the rate Facebook is growing, the question should really be who isn't? It claims to have 30 million users around the world, with 15,000 joining each day. Although it used to be open only to undergraduates and graduates from certain universities and colleges, it has recently been opened up for anyone to join, and it is no longer the preserve of the 18-24 age group.

A survey by analysts at comScore Media Metrix found that the most dramatic growth in the use of social networking sites was among 25- to 34-year-olds, and the number of users aged 35 and older had grown by 98% between May 2006 and May 2007.

Jack Flanagan, the company's executive vice-president, said: "Since the decision to open MySpace registration to everyone, the site has seen visitors from all age groups flood the site. As the overall visitation to Facebook continues to grow, the demographic composition of the site will likely more closely resemble that of the total internet audience." In other words, we may all soon be signing up.

So should we all be on it?

You could be asking for trouble. Britain's Prince William created a profile, before having to take it down after being bombarded with 'pokes' and friend requests.

And there are other downsides to joining the social networking phenomenon. Oxford University's student union urged its members to tighten their profile privacy settings last month after discovering that dons were trawling the site for evidence of postexam misbehaviour. Meanwhile, photographs of Miss New Jersey Amy Polumbo posing with pumpkins held to her chest led to an alleged blackmail and the national Miss America organisation reviewing her fitness to hold her crown. In Toronto, five students were banned from a school trip after posting disparaging remarks about their teachers. Research suggests one in five employers scour Facebook and similar websites to check up on current or prospective employees.

But it's just a bit of harmless fun, isn't it?

Not entirely. These sites can pose security risks. According to Graham Cluley at the software security firm Sophos, viruses can be spread via MySpace, and users can embed viruses in their home page.

Cluley warned users to be careful not to give away too many details about themselves on their profiles, which could leave them open to identity theft. "Often, people reveal their date of birth, the name of their pet or their wife's maiden name, " he said. "These can often be linked to sensitive bank or email passwords. Some things should be kept private."

He added: "The number of friends a person has can become like a competition. As a result, some people become friends with users they don't even know.

This creates a risk because the more you accept, the bigger the possibility that there will be a bad apple among them."

Isn't it all just a fad?

Perhaps, but the bubble shows no sign of bursting. Big business has been taking note. After Rupert Murdoch bought MySpace, other big online players have taken an interest in Facebook. On the back of a takeover offer from Yahoo! , Microsoft is now said to be interested in making a bid for the company. Jon Gibs, at the market research firm Nielsen/NetRatings, believes social networking is here to stay. He said:

"If anything, it will become more ingrained in mainstream sites, just as reality TV programming has become ubiquitous in network programming."