The social networking sites Twitter and Facebook came under a “denial of service” attack yesterday, locking out millions of users.

Under such attacks, sites are deluged with traffic in an attempt to bring them down.

It was the first time that Twitter had been attacked and the site was out of action for two hours in the afternoon. The motive remains unclear as does the identity of those responsible. Facebook users reported problems all day. It is not clear if the attacks were linked but the sites have reportedly teamed up with Google to investigate.

On Twitter’s corporate blog, Biz Stone, a co-founder of the site, said: “On this otherwise happy Thursday morning Twitter is the target of a denial of service attack. Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways and, in this case, Twitter for intended customers or users.”

The site was operating again last night, although Twitter said: “We are continuing to defend against and recover from this attack.”

Brandee Barker, a Facebook spokesman, said: “No user data was at risk and we have restored full access for most users. We are continuing to monitor the situation to ensure users have the fast and reliable experience they have come to expect.”

Google websites were also targeted but the company managed to deflect the assaults. “Google systems prevented substantive impact to our services,” said Nate Tyler, a spokesman for the company.

Graham Cluley, of the internet security specialist Sophos, said on his blog: “A denial of service attack occurs when computers bombard a website with requests for information. Typically, hackers can control thousands of innocent users’ computers centrally and command them to visit a site that they wish to flood with traffic, making it impossible for other internet users to get through.

“It is a bit like 15 fat men trying to get through a revolving door at the same time: nothing can move. In the meantime, microbloggers around the world are likely to be left twiddling their thumbs.” He said that Twitter needed to build a stronger infrastructure to fight what were relatively commonplace attacks on the internet.

Colin Sweetman, who provides internet technology and digital media advice through his company E-Volutionary.net, said that to defend against “denial of service” attacks, Twitter would have had to pinpoint where the traffic was coming from.

Twitter has experienced downtime before but cases have become infrequent as stability has grown. The site has seen a rise in popularity, experiencing a 22-fold increase in traffic in the past year, according to the internet analysts Hitwise. Twitter is now the 38th most visited site in Britain.

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