Users need to be aware of the security risks before signing up for Facebook’s next generation online messaging service that includes online chat, text messages and other real-time conversation tools, computer security firm Sophos cautioned on Thrusday.

Facebook on Monday launched its new messaging system, which includes facebook.com email addresses.

Sophos particularly noted that fraudsters are increasingly using hacked Facebook accounts to send spam messages as they are more likely to be opened by recipients, who think the messages are from friends.

Users need to realise that these new features increase the attack surface on the Facebook platform, and make personal accounts all the more alluring for cybercriminals to break into, Graham Cluley, Sophos senior technology consultant, is quoted as saying in media reports.

According to Facebook founder Mark Zuckerberg, almost 350 million of Facebook's more than 500 million members send messages using its existing service, with more than 4 billion digital missives sent daily.

Users also need to be aware that Facebook will be storing a complete archive of all their communications with one person. This raises concerns as to how this data could be misused if it fell into the wrong hands, added Cluley.

In a press statement, Sophos said that Facebook accounts will now be linked with many more people in the users' social circles - opening up new opportunities for identity fraudsters to launch attacks. 

Sophos also warned Facebook users about their accounts, password and new application selection.

However, despite the security threats, according to market watchers, free personalised facebook.com email service is a challenge to the established e-mail giants, such as Microsoft's Hotmail, Yahoo! Mail and Google's Gmail.