Facebook became the most popular site in the US for the first time last week, pushing Google into second place although the search engine giant remains dominant in Britain.
The social networking phenomenon, set up by Mark Zuckerberg from his dorm in Harvard in 2005, secured 7.07 per cent of website hits from the US during the week ending 13 March, according to the market research group Experian Hitwise.
While it had been the most popular site on Christmas Eve and Christmas Day, this is the first time it has overtaken Google for an entire week, as the search engine fell to second place with a 7.03 per cent share.
Facebook's weekly hits were up 185 per cent year on year against Google's rise of 9 per cent. The data did not include hits on the search engine's video-sharing website YouTube, Google Maps or its email service Gmail. Hitwise said that while the two companies do not directly compete, Facebook's ascent to the most popular US website was an "important milestone".
Robin Goad, a research director for Experian Hitwise, said: "Facebook's growth around the world has been astonishing and it is now the dominant social network in a number of key markets."
This will be worrying for Google, especially after its attempts to enter the social networking space with the launch of Google Buzz have struggled. The group has added new features to its search engine to drive growth, including voice search and translation services, while it is working on a picture recognition service called Google Goggles. Yet Google remains the most popular website in the UK, which Mr Goad put down to the even more dominant position it holds than in its home market.
Over the past week, Google took 9.4 per cent of the share of hits in the UK, while Facebook was the second most-visited site, accounting for 6 per cent.
Mr Goad said: "It is a different story in the UK, because of Google's hold on the search market. It is very unlikely that Facebook will overhaul its lead in the market for a while yet."
He said that early growth rates in Facebook were higher in the UK where it was immediately marketed as a general social network. "While it was set up in the US, it was initially associated with students, and it had significant competition in the form of MySpace. The penetration relative to population has now caught up."
Facebook has experienced extraordinary growth, and revealed it had more than 400 million users in February. It overtook MySpace for the first time in June 2008. There are still question over the group's business model as it struggles to turn its huge user base into profits. Google, on the other hand, generated revenues of $23bn last year, predominantly through its search advertising business.
Mr Goad said: "The growth may be slowing but it is not tailing off." He said an indicator of when a site is struggling is when users spend less time on it. "It happened with MySpace. Session times peaked at 25 minutes, when that dropped, it marked when the site began to lose popularity." Users currently stay on Facebook for 30 minutes a day in the UK, which is not dropping. "I don't see a decline. There is no threat to Facebook in the way there was for MySpace," Mr Goad said.
"Facebook came along at the right time. It has become the defacto social network, the only one to gain mass market acceptance."