Facebook has suffered its first fall in UK users, with a 5% drop between December and January, according to new figures.

However, Facebook still had 8.5 million unique users in January and remains the most popular social networking website in the UK, according to Nielsen Online, the internet research company behind the results.

And Facebook's nearest rival, MySpace, also saw a 5% drop in UK traffic between December and January, according to Nielsen Online.

MySpace ranked second in terms of UK social networking websites in January, with 5 million unique users, while Bebo, which saw users drop by 2% month on month, was ranked third with 4.1 million.

Facebook shed around 400,000 UK unique users month on month, following an impressive 17 consecutive months of growth, according to the research.

This is the first drop Nielsen Online has recorded in Facebook's user numbers in the UK.

Nielsen Online has been recording the number of monthly unique users visiting Facebook since July 2006, when the social networking website's UK audience first became large enough to be regularly measured by the company's research panel.

"One month of falling audiences doesn't spell the decline of Facebook or social networking," said Alex Burmaster, European internet analyst at Nielsen Online.

"However, most of the leading social networks are less popular in the UK than they were a year ago. It was inevitable that early growth rates couldn't be sustained and the larger networks have been plateauing over the last few months."

To put this dip in context, Facebook's audience is still a massive 712% higher than in January 2007 and 9% bigger than at the end of October.

Over the same period MySpace has seen its number of unique users fall by 9% since January last year and 14% over the past three months.

Bebo has seen a 53% year-on-year increase in unique users compared to January 2007, but a dip of 8% since the end of October.

"Growth among the big players looks to be more about getting people from their competitors, not attracting new people to social networking," added Burmaster.

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

Nielsen Online only measures website traffic based on a panel of UK users at home and work - it does not cover usage in schools, universities and internet cafes, meaning that younger internet users are under-reported.

Other online traffic research firms, such as Comscore, do include users outside work and home, and as a result their unique user figures for individual websites tend to be significantly higher.

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