The popularity of Facebook in the UK may have reached its peak, according to figures showing that the number of unique visitors to the site has fallen for the first time.

Research firm Nielsen Online reported that Facebook showed a five per cent decline from 8.9 million users in December to 8.5 million last month after 17 monthly consecutive increases.

High-profile members of the social networking site, including London mayoral candidate Boris Johnson and actress Sienna Miller, could be giving Facebook a credibility problem, according to industry watchers.

"This fall is a significant moment in the development of Facebook, and potentially marks the high water mark of the site's popularity in the UK," said Nic Howell, deputy editor of internet trade magazine New Media Age.

"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."

However, other industry experts believe that the decline could simply represent a seasonal dip for social networking sites as a whole.

Rival MySpace UK also experienced a five per cent fall in user numbers, from 5.3 million to 5.1 million.

Facebook's audience is still 712 per cent higher than a year ago and nine per cent higher than three months ago.

"Just as one swallow does not make a summer, one month of falling audiences does not spell the decline of Facebook or social networking," said Alex Burmaster, European internet analyst at Nielsen Online.

Tim Johnson, chief analyst at Point Topic, believes that the figures represent a cooling down in terms of the novelty of social networking, but that will usage will continue.

"The first wave of adopters are cooling off a bit as the initial craze starts to fade, but people will keep on using social networking sites even though they spend less time on them," he said.

Although MySpace and Facebook are responsible for over 80 per cent of the market, the range of sites will become much more diverse, according to Point Topic.

Growth in numbers will resume as the idea spreads to older age groups and poorer people, although it will never repeat the speed of the initial viral infection.