NEW YORK — A hacker attack Thursday shut down the fast-growing messaging service Twitter for hours, while Facebook experienced intermittent access problems.
Twitter said it suffered a "denial-of-service" attack, in which hackers command scores of computers to a single site at the same time, preventing legitimate traffic from getting through.
For Twitter users, the outage meant no tweeting about lunch plans, the weather or that Twitter was down.
The Twitter outage began about 9 a.m. and lasted a few hours.
Denial-of-service attacks are typically carried out by "botnets" — armies of infected computers formed by spreading a computer virus that orders compromised machines to phone home for further instructions. They are generally used to send out spam or steal passwords, though some can be commanded to overwhelm Web sites.
The fact that a relatively common attack could disable such a well-known Web site shows just how young and vulnerable Twitter still is, even as it quickly becomes a household name used by celebrities, large corporations, small businesses and even protesters in Iran.
"Clearly, they need a stronger infrastructure to be able to fight this kind of attack," said Graham Cluley, senior technology consultant at computer security firm Sophos.
The entire site being down means Twitter hasn't put enough measures in place to prevent such an attack, Cluley said. That could include working with Internet service providers to filter potentially malicious requests from legitimate ones, as well as having servers spread out around the world.
Facebook, whose users encountered intermittent problems Thursday morning, was also the subject of a denial-of-service attack, though it was not known whether the same hackers were involved. Unlike Twitter, Facebook never became completely inaccessible. Facebook said no user information was at risk.
Technology business analyst Shelly Palmer told AP Radio that denial-of-service attacks are a reality of the information age.
"People tend to want to take sites that are very public and go after them," said Palmer, managing director of Advanced Media Ventures Group. "In fact, you'd be surprised how many sites for major companies are really attacked on a daily basis. This is a crime, it's a real crime and it should be treated that way."
Earlier this week, Gawker Media, which owns the eponymous media commentary blog and other sites, was also attacked. In a blog post, Gawker said Tuesday it was attacked by "dastardly hackers," leading to server problems that caused networkwide outages Sunday and Monday. It was not immediately clear whether those attacks were related to Twitter's.
Thursday's was not the first — and likely not the last — outage for Twitter.
Besides planned maintenance outages, overcapacity can cripple Web sites, especially such fast-growing ones as Twitter and Facebook.
According to comScore, Twitter had 20.1 million unique visitors in the United States in June, about 34 times the 593,000 a year earlier.