BBC NEWS | Technology | Twitter site reopens after attack
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.