The loss equates to 400,000 users from December to January, but audience is still 712% higher than it was a year ago and 9% higher than three months ago.
It follows numerous articles in the media about people tiring of social networking sites and Facebook in particular.
However, Facebook denies experiencing a drop in their unique users. The website said it has seen a rise in active users month on month and currently has 8.3m active users in the UK, which is up by around 600,000 on the 7.7m users recorded in December.
Facebook said it tracks active monthly users, rather than registered users or unique visitors, to reflect those who have used the site in the past 30 days.
The website still holds on to top spot among the social networking sites in the UK, remaining above and , which also suffered a decline in popularity last month.
MySpace's audience fell 5% from 5.3m unique users in December to 5m users in January. The News Corporation-owned site is down 9% year on year and down 14% from three months ago.
Bebo, which remains in third place, dropped 2% in January to 4.1m unique users. This figure is up 53% on a year ago but is down 8% from three months ago.
Alex Burmaster, European internet analyst at Nielsen Online, said: "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.
"It was inevitable that the early growth rates couldn't be sustained and the larger networks have been plateauing over the last few months."
He added that growth among the big players looks to be more about getting people from their competitors not attracting new people to social networking.
He said: "Real growth potential lies in the niche networks -- those based on a particular lifestyle or interest, such as travel, music, wealth or business."
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Further social networking sites rounding out the top 10 in the UK include , , , , , and .
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