SAN FRANCISCO - Twitter and Facebook said they suffered service problems from hacker attacks on Thursday, raising speculation of a coordinated campaign against the world's most popular online social networks.
Twitter, the popular micro-blogging service, was knocked down by a malicious attack that prevented people from accessing its website for several hours on Thursday.
Facebook members saw delays logging in and posting to their online profiles, which the social networking site said was related to an "apparent distributed denial of service attack."
Facebook was working with Twitter and Internet search company Google Inc to investigate further, said a person familiar with Facebook but who was not authorized to speak to the press.
Speculation swirled on the Internet that other social networking sites had also come under attack, after relatively lesser-known site LiveJournal said it too had been targeted by hackers on Thursday. But those rumors could not be confirmed.
The incidents follow a wave of similar cyber attacks in July that disrupted access to several high-profile U.S. and South Korean websites, including the White House site. South Korea's spy agency said at the time that North Korea might have been behind the attacks.
Twitter co-founder Biz Stone said on Twitter's blog that the site was the victim of a denial-of-service attack, a technique in which hackers overwhelm a website's servers with communications requests.
"We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate," Stone wrote.
A separate Twitter status Web page said later on Thursday that the site was back up, but that Twitter was continuing to recover from the attack.
Google said in an emailed statement that it was in contact with some non-Google sites that were impacted by Thursday's attacks to help investigate.
"Google systems prevented substantive impact to our services," the statement said.
Motives for denial-of-service attacks range from political to rabble-rousing to extortion, with criminal groups increasingly threatening to hobble popular websites that don't pay demanded fees, according to security experts.
Twitter's newfound fame makes it an easy target for hackers, said Steve Gibson, the president of Internet security research firm Gibson Research Corp.
Twitter, which lets users publish short, 140-character messages to groups of online "followers," is one of the fastest-growing Internet companies.
The number of worldwide unique visitors to the Twitter website reached 44.5 million in June, up 15-fold year-over- year, according to comScore data.
Security experts said a single group could have been behind the problems on Twitter, Facebook and the other sites as hackers evolve their ability to attack multiple sites at once.
"History would tell us that it's probably the same attacker or group of attackers that is launching both attacks," said Kevin Prince, the chief technology officer of security services provider Perimeter eSecurity.
A representative for blogging website LiveJournal said the site was also affected by a cyber attack for about one hour on Thursday morning.
While the company "can't be 100 percent sure that it was the same attacks as on Twitter, Facebook, et al, it would be a huge coincidence if they aren't tied to one another," said LiveJournal representative Tim Smith.
A denial-of-service attack on Twitter would be particularly effective since "it's going to be very visible to a huge population of people who have now, to some degree, become dependent on this next-generation, real-time service," said Gibson.
Some Twitter users appeared to be taking the incident in stride.
"It's just an annoyance. Remember Twitter was down in 2007 and 2008 all the time," said Robert Scobble, a commentator on the technology industry who boasts 93,000 "followers" on Twitter, referring to a period when Twitter's rapid traffic growth occasionally led to several service disruptions.
For lawyer Zabi Nowald, it was just another day -- Twitter or no Twitter -- as he headed to work in downtown Los Angeles with a laptop in one hand and a Blackberry in the other.
"None of my friends do Twitter; none of my employers do," said Nowald, 27. "It affects my life zero. I lost something I never had."