Facebook has overtaken MySpace for the first time in the UK in terms of the number of unique visitors, according to new research.

Figures from Nielsen/NetRatings suggest that Facebook received 6.5 million unique visitors in August, compared to 6.4 million for MySpace. Both sites are now visited by one in five UK surfers.

Facebook's audience has grown by 541 per cent since December 2006, compared to 20 per cent for MySpace.

Almost half of Britain's online community of 15.3 million people visited at least one of the 10 most popular social networks in August, demonstrating the extent to which the phenomenon has penetrated the online world.

Alex Burmaster, European internet analyst at Nielsen//NetRatings, said: " Three months ago I predicted that Facebook would catch MySpace in September this year, but its continued growth meant it only took until August to achieve this feat.

"This is particularly extraordinary considering that, at the end of 2006, Facebook was one fifth the size of MySpace.

"However, even in the fickle sphere of social networking, MySpace has managed to maintain more than six million visitors in each of the past six months, and it will be interesting to see if Facebook can achieve this level of audience retention."

Nielsen//NetRatings also identified significant competition from PerfSpot.com, which had no recorded figures until April, but has grown by 756 per cent in just four months.

This makes it the fastest growing social network in 2007 ahead of Facebook, which has grown by 541 per cent in the eight months from December 2006 to August 2007.

"The suspicion that the next big thing in social networking could always be just round the corner is illustrated by PerfSpot," said Burmaster.

"It was not even on the social network radar until April of this year, but in the last four months its visitor numbers have grown at a greater rate than Facebook across the last eight months."

Some observers have suggested that the future of social networking will revolve around specific interest groups, as opposed to the general behemoths that dominate today.

"The fact that a number of the fastest-growing networks concern specific interest areas, such as business (LinkedIn), travel (WAYN) or music (Imeem), seems to add credibility to this theory," said Burmaster.

The analyst pointed out that it is necessary to look away from the 'Big Three' to find the most engaging social networks.

The average Second Life visitor spends an astonishing five and a half hours a month in the virtual world, over twice as long as the average visitor to Facebook and almost four times as long as the average visitor to MySpace.