Twitter has 10m active users in the UK

"Twitter now has 10 million active users in the UK, out of 140 million worldwide, the company has revealed – and 80%, an unusually high number, access it through mobile phones.
The data about the UK, revealed for the first time by the company, indicates that 8 million who logged in to the company's site in the past 30 days did so from a mobile phone, compared to Twitter's global average of 55%.
Independent studies suggest that the UK is the fourth-largest country for Twitter users in the world, after the US, Brazil and Japan.
The UK's proportion of mobile users is 45% higher than the average for the world, indicating the strength of Twitter use in the UK. Although most users are almost certain to log in via a desktop or laptop computer at some point, a Twitter UK spokeswoman said: "We have seen growth in the number of people signing up from a mobile."
That, she said, went back to the DNA of Twitter – which was originally set up in 2006 as a service that would work over SMS, which is why its messages are limited to 140 characters.
Twitter also points out that its users are particularly active in generating content: 60% have contributed to the network, either through tweeting or posting a picture or other content, compared to the world's biggest video site YouTube, where sources say just 1% of users ever posts a video."
Source:  The Guardian, 15th May 2012