The security firm Sophos has published an FAQ on security issues presented by Facebook's new and weirdly still unnamed messaging system.
Could I receive spam and malware via the new system?
Yes. The new features do increase the attack surface of the Facebook platform, and make the accounts of users all the more alluring for cybercriminals to break into.
Facebook accounts will now be linked with many more people in your social circle - opening up new opportunities for identity fraudsters to launch attacks. Furthermore, because Facebook will be storing a complete archive of all of your communications with one person - there will be concerns as to how such data could be misused if it fell into the wrong hands.
It will be critical for Facebook to implement more effective filtering mechanisms to prevent fraudsters from manipulating Facebook users into falling victim to new spams, scams and phishing attacks.