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.