According to this report at Paid Content it is:
Web measurer Hitwise says it’s the reverse – at least, proportionally. “Last week the site’s share of UK internet visits was 70 percent higher its share of visits in America,” said analyst Robin Goad. ”Twitter cannot yet be considered mainstream in the USA, but in the UK it’s getting there.” Its growth here – 485 percent so far this year and 631 percent year-on-year – is slower than the eightfold annual international increase, though.
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They say: TweetDeck 
