LONDON: Facebook, the social networking service, has overtaken Yahoo to become the third biggest web property by unique audience in the UK, and has also grown at a faster rate than the top-ranked portals run by Google and Microsoft, comScore, the online measurement firm, reports.

According to comScore's figures, Facebook received a total of 23.4 million visitors from the UK in April this year, amounting to an increase of 63% in user numbers on an annual basis.

This compared with a total of 33.5 million for Google's websites, up by 3.2 million on April 2008, with Microsoft seeing growth of 1.1 million, to 29.4 million, over the same period.

Yahoo also saw an upturn in visitor numbers of 8% year-on-year, but also posted a decline of 1% in its audience size quarter-on-quarter to 23.1 million, and thus fell behind the UK's biggest social network.

eBay, the online auction service, was in fifth place in terms of unique users, with 21.9 million, while the BBC's online properties posted a small expansion in visitor numbers to 19.2 million, and AOL registered a rise of five million, to 15.9 million in all.

Another study from comScore also stated that Facebook published the highest number of online display ad of any UK website in April this year, hosting 21.6% of all executions during the month.

Jamie Gavin, a marketing communications analyst at the research firm, argued that Facebook is "by far the most successful social network."

As a result of its growing dominance, its competitors "may have to become more niche to survive, such as MySpace focusing on music. Taking on a social media behemoth like that would be a tough job.”

Robin O'Neill, head of online trading at GroupM, WPP's Group's media arm, said the number of visitors to Facebook hasn't yet translated into ad revenues, but that this situation may change.

He added that "there's a lot of potential for growth as brands are coming around to that way of thinking," even if the current adspend to audience ratio was "not even close".

In particular, the site "hasn't had the amount of money spent” that could be expected because, as yet, advertisers "haven't got to grips with its speed of growth."

Ian Maude, head of online at Enders Analysis, the analysts, similarly said Facebook "is still taking a small share of online spend", but "the gap will shrink as brands seek better advertising and targeting is improved."

Data sourced from New Media Age; additional content by WARC staff