June was the worst ever month for attacks on social networking sites, due in large part to the Koobface worm, according to security specialist Kaspersky.
As social networking sites grow in popularity, they are becoming an ever more popular target for cybercriminals, Kaspersky said.
"June 2009 marks an important milestone in the evolution of social-networking malware - the activity we've seen this month exceeds by far any other month in the past," Kaspersky malware researcher Stefan Tanase said.
Hundreds of new variants of malware such as the Koobface worm have emerged, targeting an ever-increasing range of social networking sites.
The number of Koobface variants alone shot up from 324 at the end of May to almost 1,000 by the end of the month, Kaspersky said.
Tanase said the rapid growth "proves that the strategies used by cybercriminals to infect users are much more efficient when adding the social context to their attacks."
Targets of the Koobface worm now include Facebook, MySpace, Hi5, Bebo, Tagged, Netlog and Twitter.
Koobface propagates itself by using infected computers to send messages via the owners' social networking site accounts. The messages link to a fake YouTube-like page which invites users to download a "new version of Flash Player," but instead downloads the worm into victims' machines.