The UK ranks as the 6th most-active territory in the world, but a report from Czech start-up Socialbakers has suggested a loss of over half a million users in Britain over Christmas. Facebook have moved quickly to deny the findings.

The social metrics report gained further publicity in the Guardian; with the newspaper speculating this drop was due to market saturation of the network, over half of the UK population (around 33 million people) using Facebook.
That user base (33m) 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.
The growth of Facebook in the developing world would more than make up a small loss in the UK where exposure and saturation are perceived to have peaked. However, Facebook have officially attempted to debunk the reports with the below statement.

“From time to time, we see stories about Facebook losing users in some regions. Some of these reports use data extracted from our advertising tool, which provides broad estimates on the reach of Facebook ads and isn’t designed to be a source for tracking the overall growth of Facebook. We are very pleased with our growth and with the way people are engaged with Facebook – more than 50% of our active users log on to Facebook in any given day.”
  
With Zuckerberg's team denying the loss of users, Socialbakers have been left a little red-faced by the British press' bold headlines, and their CEO Jan Razab has since backtracked somewhat on their blog.

"Facebook measures by Monthly Active Users...The numbers are from Facebook’s ad interface, and Facebook unlike other companies, updates this data on a pretty regular basis. The monthly active user count is statistically vulnerable to more casual users of the platform, users that don't use it that often and might fall out of the 30-day range from time to time. What we figured out after working with this data for a long time is that Facebook’s ad interface has a slight delay, typically by several weeks."

 So have Facebook actually lost users in the UK? Perhaps yes, though perhaps the metrics are still being counted and the Guardian slightly sensationalised some preliminary reports - for which Socialbakers have hastily clarified. What we do know if that Facebook is growing massively in India and Brazil (whatever incremental percentage decreases in the US and UK) and that the Facebook team were keen to avoid any Instagram-style PR disaster following policy change - where users left the service in their droves.