Facebook users are being warned about a new application on the social networking site that contains adware.

'Secret Crush' contains a download of the Zango adware program which automatically sends itself to five friends.

It has already infected three per cent of Facebook users, over one million computers, according to security firm Fortinet.

"People are now developing Facebook 'platform applications' for profit rather than just for fun, but this does not mean that all widgets are going to be malicious," said Guillaume Lovet, manager of Fortinet's European Threat Response Team.

"Honest ways to generate profits exist on Facebook, but users must use common sense and protection to avoid being scammed and abused."

Lovet warned that social networking sites are fast becoming dangerous places, and that users who are unaware or run unpatched browsers are increasingly at risk.

The adware has spread quickly because people are more willing to install new applications on Facebook, something they are learning not to do with email attachments.