LONDON - Facebook overtook Microsoft websites in Britain for the first time last month, becoming the UK's second-most popular after Google as people aged over 50 flocked to social networks, online measurement body UKOM/Nielsen said.

Facebook attracted a record 26.8 million visitors in Britain in May, up 7 percent year on year, beating the 26.2 million who visited Microsoft's MSN/WindowsLive/Bing sites combined, the organisation said on Monday. Google had 33.9 million.

Twitter's UK audience jumped by a third to 6.1 million, after thousands of users retweeted allegations of celebrity scandals in defiance of gagging orders, including an extra-marital affair by Manchester United soccer star Ryan Giggs.

UKOM/Nielsen said the number of women pensioners visiting the site doubled after "Giggsgate".  

"The growth in audiences to these social networks is now primarily being driven by the 50-plus age group. Just a few years ago, this group may have found itself out of place on these sites," UKOM general manager James Smythe said.

He said over-50 year-olds accounted for more new adults visiting Facebook in the last two years than under-50s, resulting in an age profile far more closely reflecting that of the UK online population as a whole than previously.

Older age groups were also more likely to visit Twitter than in the past, but under-18s were less likely to visit the site than two years ago -- which was not the case for Facebook.

Business network LinkedIn , whose market value has risen 58 percent to $6.65 billion since its New York stock market debut last month, registered 3.6 million UK visitors in May, up 57 percent from a year earlier.

Elsewhere, Facebook attracted 140 million visitors in the United States, up 12 percent. In Spain its numbers were up 7 percent, in France 18 percent, in Italy 26 percent and in Germany 72 percent.

Twitter's visitor numbers rose 22 percent in the United States, 48 percent in France, 58 percent in Italy and more than doubled in Spain. But in Germany they fell by 11 percent.

UKOM/Nielsen monitored the online behaviour of about 50,000 people in Britain and similar numbers in the other countries. The panel was recruited both online and offline.