According to data from Nielsen Online ace social networking site Facebook has suffered its first drop in UK users, with users falling 5% to 8.5 million in January 2008 from 8.9 million in December 2007.

MySpace users fell by 5% in January, while Bebo's audience dropped 2%. Meanwhile, niche social networking sites saw an increase in traffic.

This is the first drop in Facebook user numbers since July.

Nic Howell, deputy editor of industry magazine New Media Age, told the BBC that the site is 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.

The top 10 UK social networks are (according to Nielsen Online):

1) Facebook

2) MySpace

3) Bebo

4) Windows Live Spaces

5) BBC Communities

6) Friends Reunited

7) Yahoo! Groups

8) MSN Groups

9) Google Groups

10) Piczo

However, Nielsen's figures do show 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," said Alex Burmaster, European internet analyst at Nielsen Online.