Microblogging site Twitter has been by a denial-of-service attack that rendered the site unavailable for several hours.

In a status report Twitter reported that the site was back up, but users still were having trouble reaching it. The site itself was down for about two hours before it resumed service, although Twitter remained under attack and warned users in another status update that as it recovered, users would experience "some longer load times and slowness," as well as network timeouts.

In its official blog, Twitter said that no user had been compromised and thanked its partners for getting the service back to normal. "Over the last few hours, Twitter has been working closely with other companies and services affected by what appears to be a single, massively coordinated attack. As to the motivation behind this event, we prefer not to speculate," the blog said.

A DoS attack is an attempt to make a website or service unavailable to intended users by flooding the service or site with incoming data requests, such as e-mails. Motives for DoS attacks vary, but perpetrators mostly target companies with high-profile, highly trafficked websites, and usually there is some kind of financial motivation for the attack.

Graham Cluley, a senior technology consultant with security software vendor Sophos, said it's unlikely money is the motive here, since Twitter does not have much of its own to part with because the business is not yet profitable.

DoS attacks also can be politically motivated, he said, and while some countries' governments don't like Twitter - notably, Iran - he doubted the attack was politically motivated. "It's most likely to be a teenager in a back bedroom somewhere showing off," Cluley said.

When a site is hit with a DoS attack, administrators will try to distinguish between valid requests to access the site and malicious ones, and redirect the malicious ones to another domain if possible, he said. As Twitter's site was up and running a couple of hours after the attack, it's likely the company was able to do this, or the hacker may have simply ended the DoS attack, Clulely said.

In just three years, Twitter has become an enormously popular Internet service with about 30 million unique users and counting. In addition to being a social tool for people to share constant status updates about their activities, it also has become a tool for journalists, public relations specialists, businesses and public figures to share information with millions of users.

Like Facebook and Google, Twitter also has become an integral part of U.S. popular culture, with the slang word for posting something on Twitter, "tweet," becoming part of U.S. English vernacular.

Twitter is no stranger to outage problems, although it had been starting to improve its availability level in the past year. According to a report by Pingdom released in February, Twitter recorded 84 hours of downtime in 2008, but 84 percent of that was in the first half of the year. The site finished 2008 with uptime of 99.04 percent, which still lagged behind other popular social-networking sites like Facebook and MySpace.