San Francisco - Hackers knocked wildly popular micro-blogging site Twitter offline for several hours Thursday and caused performance stumbles at hot social-networking service Facebook.
Twitter was down for more than two hours before engineers at the California firm were able to get it back online with a warning at the website that "we are continuing to defend against and recover from this attack".
"On this otherwise happy Thursday morning, Twitter is the target of a denial of service attack," Twitter co-founder Biz Stone said in an official company blog.
"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," he said.
Facebook service was "degraded" by an early-morning distributed-denial-of-service attack on the Palo Alto, California-based Internet star's website, said Facebook spokeswoman Brandee Barker.
"No user data was at risk and we have restored full access to the site for most users," Barker said late Thursday morning.
"We're continuing to monitor the situation to ensure that users have the fast and reliable experience they've come to expect from Facebook."
Twitter and Facebook have reportedly teamed with internet powerhouse Google to investigate the attacks.
Hackers evidently employed classic distributed-denial-of-service (DoS) attacks in which legions of zombie computers, machines infected with viruses, are commanded to simultaneously visit a website.
Such massive onslaught of demand can overwhelm website servers, slowing service or knocking it offline.
The DoS attack hit Twitter about 6 am local time (1200 GMT) and caused the service to be offline until nearly noon.
Access to the website continued to be slow, with some aspiring users getting messages telling them that connections had "timed out" because Twitter computers were taking too long to respond.
After the service resumed, Twitter user Benjamin Hobbs fired off a message saying he "wishes the Denial-of-Service idiots would get a life and leave Twitter alone."
While an everyday chatting tool for many, Twitter has become a weapon used by dissidents to circumvent censorship in places where freedom of speech is suppressed.
Independent information about deadly riots in China's remote northwest filtered out on Twitter, YouTube and other Internet forums in July, frustrating government efforts to control the news.
The communist authorities who built the so-called Great Firewall of China raced to stamp out video, images and words posted by Internet users about unrest which, officials said, left at least 140 people dead.
Similar to the phenomenon seen a month earlier during Iran's political turmoil, pictures, videos and updates from Urumqi poured onto social networking and image sharing websites such as Twitter, YouTube and Flickr.
In many cases, items were reposted by other Internet users on sites outside China to preserve the content, while Twitter helped link people around the globe to images Chinese authorities did not want seen.
Cyber-sympathizers from around the world joined forces through Twitter in June to help Iranian protestors dodge censorship, get out news of violent clashes and avoid real-world capture following Iran's disputed election.
Cyber attacks on webpages of Iranian opposition figures have continued in the aftermath of the controversial presidential election in Iran.
- AP