Cyber criminals have managed to inflitrate Facebook and are targeting social networkers.
Nigerian fraudsters are using accounts to send messages to groups of friends pretending that the sender is abroad, in trouble and needs money.
The warning comes after one Karina Wells from Sydney, Australia received a Facebook message from one of her friends saying he was stranded in Lagos, Nigeria, and needed $500 for a ticket home.
Mrs Wells was suspicious but said that the English was good apart from the use of "cell phone" instead of "mobile phone".
She went along with the instructions, got the Western Union details of the fraudster then passed the details onto Facebook.
"Naturally I was concerned as, to all intents and purposes, this seemed to be legitimate", Mrs Wells told The Telegraph.
Security experts have now revealed that the con was made possible by a password stealing virus that gave the fraudsters access to Mrs Well's friend's account.
A Facebook spokesman said, however, that the problem isn't widespread: "Only a small percentage of users have been affected by recent spam attacks. We are updating our security systems".