Twitter and Facebook, two of the Web's hottest hangouts, suffered service problems Thursday, raising speculation that they had come under a pre-planned coordinated attack by hackers.
Twitter, the popular micro-blogging service, was knocked down by a malicious attack that prevented people from accessing its website for several hours Thursday.
Facebook members saw delays logging in and posting to their online profiles. The social networking site is working with Twitter and Google to determine whether there was foul play, a person familiar with the company said.
The Twitter outage follows 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: "As we recover, users will experience some long load times ... We're working to get back to 100 percent as quickly as we can."
The time stamp was listed as "3 hours ago," as of 2:05 p.m. New York time.
Another social media site, LiveJournal, also experienced trouble, CNBC learned.
And just this past Sunday and Monday, the popular Web sites run by Gawker Media went off the grid after an apparent attack by hackers. The attack was aimed at Consumerist.com, but the rest of the Gawker sites were also shut down. Gawker sold Consumerist to Consumers Union, the nonprofit group that publishes Consumer Reports magazine, late last year, but Gawker still hosts the site.
Gawker’s sites were back working on Tuesday.
Social Media, Antisocial Attacks
Twitter's newfound fame makes it an easy target for hackers, said Steve Gibson, the president of Internet security research firm Gibson Research.
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.
Denial-of-service attacks are "not different than any other attack, but it's going to very visible to a huge population of people who have now, to some degree, become dependent on this next-generation, real-time service," he said.
A single group could have been behind the problems on Twitter and Facebook as hackers evolve their ability to attack multiple sites at once, Gibson said.
"It seems a little too coincidental. They're like related services. They're similar. They're very popular," he said, but he pointed out it was just speculation so far.
Not everyone was concerned. 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."
—CNBC's Julia Boorstin and Jim Goldman contributed to this report.