Facebook, the UK's most popular social networking site, has suffered its first monthly drop in visitor numbers, according to figures published today.

After 17 successive months of increasing its monthly audience in the UK, Facebook suffered a 5 per cent dip in visitors between December and January - from 8.9 million to 8.5 million.

In January, Facebook's audience was still 712 per cent greater than in the same month last year, and up 10 per cent on the previous quarter, but the figures will nonetheless represent a blow to the firm, which has enjoyed a meteoric rise in subscriber numbers since becoming widely available in late 2006.

Facebook, which has become popular with a professional audience both as a way of sharing content as well as hunting for job opportunities, was not the only social networking site to suffer, however.

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MySpace, the UK's second most popular site in the category, also fell 5 per cent to 5 million, while Bebo, the third largest, dropped 2 per cent to 4.1 million, according to figures published by Nielsen Online.

In the past 12 months, several of the top ten networking sites have found that their visitors have deserted them. Networks on Yahoo! and Google-owned sites experienced falls of 16 per cent and 30 per cent respectively, while Piczo, a site specifically targeting teenagers, lost 56 per cent of its audience - suggesting that 2008 may be a year in which a general 'malaise' about social networking sets in.

"Facebook was never going to be able to carry on growing the way it has, and a lot of people - especially those who've been using it heavily - are now starting to get Facebook fatigue," Alex Burmaster, an internet analyst at Nielsen Online said.

"I think when something explodes like that a lot of people check it out because they feel they should, but while getting alerts about what your friends are up to is exciting for a time, that's inevitably going to die down."

Mr Burmaster said that the broader social networking sites would plateau this year, and that growth would come in niche sites, which were more sustainable because audiences tended to more highly engaged in subjects that were close to them.

He cited the example of WAYN (Where Are You Now), a travel networking site whose UK audience has grown 25 per cent in the past year to 461,000, and LinkedIn, the professional network, which jumped from 161,000 to 433,000.

Rebecca Jennings, an analyst at Forrester, said it wasn't surprising that Facebook's audience had dropped slightly because the surge of media interest in the site last year had led to many registrations by people who were never likely to use it in the long term.

She added that the figures only represented one month, and while social networking was "no longer in the heady days" of double or triple digit growth, sites like Facebook still had the potential to increase their audiences - both in the UK and in less developed markets.

Mr Nielsen added that the drop in Facebook's numbers between December and January could not be attributed to a lull in communication as people recover from the excesses of the Christmas season.

In the same period last year, MySpace's audience grew by 4 per cent, and Facebook's by 3 per cent, he said.

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