No other city network on the site, including New York and Tokyo, has as many members.
The total number of residents signed up to in the UK has now reached 8m.
Social media experts warn that it is now more important than ever for organisations to protect their brand reputation on social media outlets.
Henry Elliss, head of social media at Tamar, which advises brands on how to manage their reputations online, said: "Social networking sites like Facebook have clearly become an intrinsic part of British culture, much in the same way as Google.
"High profile cases of brands acting based on social network activity have demonstrated the growing power of social media -- the Facebook campaign that led to Cadbury's bringing back Wispa for example.
"As a result of this power, when it comes to brands liaising with social media consumers, the risks are high but so are the rewards if it is done well."
Many brands have begun running viral campaigns on Facebook, including the BBC, which launched an online interactive football game just over a week ago, to support its coverage of the 2008 Africa Cup of Nations. will run on Facebook until the tournament ends on February 20.
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