A computer hacker has advertised for sale what appear to be the passwords and login details of 1.5 million users of the social networking site, Facebook.
The details are being offered on a website in Russia, in groups of 1,000 for up to $45 a batch.
It is feared the information could be used for crime, including identity theft and sending millions of unwanted emails.
The BBC's Rupert Wingfield-Hayes is in Moscow.
Dan Tynan is a contributing editor to PC World Magazine.
First broadcast 29 April 2010