SAN FRANCISCO, (Reuters) – Twitter and Facebook said  they suffered service problems from hacker attacks yesterday,  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 yesterday.

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 <GOOG.O> 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 yesterday. 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 yesterday  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 yesterday’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.