After growing its UK audience for 17 straight months, Facebook’s British unique visitor count fell five percent from December to January, Nielsen Online reports. The audience fell from 8.9 million to 8.5 million, though it’s still the top-ranked UK social network, ahead of MySpace (five million) and Bebo (4.1 million).

Facebook still shows massive 712 percent annual visitor growth after opening to non-students in autumn 2006. The stats coincided with negative publicity over identity protection and data portability but it’s highly likely the seasonal dip was the result of all those Facebook-addicted office workers being home for Christmas.

Nielsen’s European analyst Alex Burmaster 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. However, most of the leading social networks are less popular in the UK than they were a year ago. It was inevitable that the early growth rates couldn’t be sustained and the larger networks have been plateauing over the last few months. Growth amongst the big players looks to be more about getting people from their competitors not attracting new people to social networking.”