Facebook's success could make it less popular with younger people

Users fell 5% to 8.5 million in January from 8.9 million in December, according to data from Nielsen Online.

This was the first drop in user numbers since July 2006 when Nielsen began compiling data on the site.

Nic Howell, deputy editor of industry magazine New Media Age, said the site was no longer as popular among its core audience of young people.

"Social networking is as much about who isn't on the site as who is - when Tory MPs and major corporations start profiles on Facebook, its brand is devalued, driving its core user base into the arms of newer and more credible alternatives," he said.

Plateau

Facebook, along with its main rivals Bebo and MySpace, lets users set up personal web pages and communicate with each other.

TOP 10 UK SOCIAL NETWORKS Facebook MySpace Bebo Windows Live Spaces BBC Communities Friends Reunited Yahoo! Groups MSN Groups Google Groups Piczo Source: Nielsen Online

Alex Burmaster, European internet analyst at Nielsen Online, said that it was inevitable that the site's early growth rates could not be sustained and user numbers were likely to plateau.

Nielsen's figures showed that there are 712% more Facebook users than a year ago.

"Just as one swallow doesn't make a summer, so one month of falling audiences doesn't spell the decline of Facebook or social networking," Mr Burmaster said.

"However, real growth potential lies in the niche networks - those based on a particular lifestyle or interest, such as travel, music, wealth or business," he added.

The data showed that MySpace users had fallen by 5% in January, while Bebo's audience had fallen 2%.

Less popular social networking sites such as Windows Live Space, BBC Communities and Friends Reunited saw a rise in users in January, Mr Burmaster said.