Facebook has 24 million daily active users in the UK

"Facebook has published UK-specific user data, broken down for the first time into daily and monthly, and total and mobile user numbers.
The new figures come as the social network’s user base continues to shift towards mobile, and Facebook is keen to convey the shift to advertisers and media planners.
In June, Facebook claims to have had 33 million monthly active users in the UK, and 24 million daily active users.
The significance of mobile to Facebook is underlined by the 20 [daily] million users, or four out of five of the 24 million who log on each day, doing so via a smartphone or tablet computer.
A Facebook spokesperson said: "We believe that understanding who comes back at least once a month is only part of the picture. Instead, businesses should focus on people who come back online every single day, because that is how they live their lives."

Source:  Media Week, 14th August 2013
Note - Further breakdown of the figs, inc US figs here