Hackers have taken micro-blogging site Twitter off-line for several hours and caused performance stumbles at social-networking service Facebook.
Twitter was down for more than two hours on Thursday before engineers at the California firm were able to get it back online.
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.
Facebook's service was affected by an early-morning, distributed denial-of-service attack on the Palo Alto, California-based internet star's website, spokeswoman Brandee Barker says.
"No user data was at risk and we have restored full access to the site for most users," Ms Barker said.
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 a massive onslaught of demand can overwhelm website servers, slowing service or knocking it offline.
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.