Micro-blogging service Twitter and social networking site Facebook have been severely disrupted by hackers.
Twitter was taken offline for more than two hours whilst Facebook's service was "degraded", according to the firms.
The popular sites were subject to so-called denial-of-service attacks on Thursday, the companies believe.
Denial-of-service (DOS) attacks take various forms but often involve a company's servers being flooded with data in an effort to disable them.
"Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users," said Twitter co-founder Biz Stone on the company's blog.
The service was restored shortly after the blog post, but the companies have had to continue to fend off the attack.
Facebook said its service was reduced but not taken offline.
"No user data was at risk and we have restored full access to the site for most users," spokeswoman Brandee Barker told the AFP news agency.
"We're continuing to monitor the situation to ensure that users have the fast and reliable experience they've come to expect from Facebook."