The latest market research from the UK indicates Facebook is facing its first decline in unique visitors, after seventeen straight months of growth.

According to Nielsen Online, the number of British Facebook users fell from 8.9 million users last December to 8.5 million in January. This could signal a trend that users in the United Kingdom are moving to other services as many schools and companies ban the site from user access.

Further, it now appears some users -- mainly those who joined in the past year -- are simply tired of using the site for social networking purposes.

Although this marks the first drop in over a year, Nielsen warns that just one month of declining audiences doesn't mean the UK social networking scene as a whole is dying, as this could simply be a seasonal drop. MySpace and Bebo, the second and third most popular social networking sites in Britain, also saw a small decline in users over the past six weeks.

MySpace UK experienced a drop from 5.3 million users in January to 5.1 million users this month. Bebo reported 4.1 million users in January, a respectable 50% increase from last year, but still down at least 9% over the past three months.

On the up side, there are 712% more Facebook users now than at this time last year, a growth rate the service could not expect to maintain forever. BBC blogger Rory Cellan-Jones believes a number of older users tested the social networking waters with Facebook, but became burned out due to the large number of widgets and add-ons that simply did not appeal to them.

Facebook remains the fastest growing social networking service, and researchers from Forrester expect to match "the same number of registered users as MySpace in Q4 of 2008, or early 2009 given the current growth rates," Forrester Research Senior Analyst Jeremiah Owyang wrote in a blog entry last month. The ability to utilize user-created applications and widgets adds to the popularity of Facebook among younger users.

The UK market is the third biggest for Facebook, after the United States and Canada, according to the company's own statistics.