Facebook's visitor numbers fell by 600,000 in the UK in December 2012

"The number of Facebook's UK users dropped by 600,000 in December, according to data by social media monitoring firm SocialBakers.
Though representing a typical seasonal dip in use over the Christmas period, the UK was the only one of Facebook's 10 busiest territories that saw a seasonal fall, with user numbers dropping 1.86%.
On SocialBakers' index the UK ranks as the world's sixth most active Facebook user base, with more than 33 million unique users in December, although that figure duplicates users who access from multiple devices.
That user base would be equivalent to 53% market penetration last month, second only to the US with 54%. The US tops the list with more than 169 million unique users per month, followed by Brazil with 65 million and India with 63 million.
[...]
Data from comScore shows Facebook's UK monthly active users plateauing at just over 31 million between September and November, falling to 31,456,000 at the end of the three-month period."

Source:  The Guardian. 14th January 2013

Note - Social Bakers track users and brands that list the UK in their profile, so this should be both desktop and mobile users