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.

SocialBakers, a Czech-based startup backed by Index and Earlybird venture funding, has gained significant traction among big business and brands keen to assess their impact on social networks.

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.

The data chimes with speculation that Facebook is reaching saturation point among the web user population in its core markets, and that continued growth is increasingly dependent on the developing world.