Micro-blogging service Twitter was taken offline for more than two hours on Thursday in what the company believes was a co-ordinated attack.
The popular site has been subject to a so-called denial-of-service attack, according to the firm's official blog.
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.
Facebook has also said it is "looking into" possible problems with its site.
"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 company's status page suggests that the site is "continuing to defend against and recover from this attack".
A ComScore study suggests the site had around 45 million users worldwide as of June 2009.
However, as many users interact with the service through mobile phones or third-party software, the actual number of users is likely to be higher.
A spokeswoman told PA news agency that Facebook was looking into reported problems on its site as well.
Denial-of-service attacks often use networks of computers - known as botnets -under the control of hackers.
The strategy is often employed by protestors against, for example, government websites, said Roger Thompson, chief research officer at security firm AVG.
"Twitter has become a poster-child for our always-connected, always-on internet culture," he told BBC News.
"With the eyes of the world's media all trained on Twitter at the moment, those behind this latest attack may be using it as a means of highlighting the vulnerability of the sites we take for granted.
"There is no profit to be made from DOS and those who do carry out an attack like this will lose their botnet, showing there is no gain to be had."

http://news.bbc.co.uk/1/hi/technology/8188201.stm