The BBC's Click program reports how easy it is to create and distribute malware through FaceBook. Evidently, FaceBook users can set up and share applications. This capability is intended to encourage them to share benign utilities, games etc. I expect you can see where this is going ... If so minded, users can write and share applications that access supposedly private information on the users' profiles, and send it out by email or whatever. Handy for budding identity thieves. Not so handy for FaceBook users who care about their privacy although, to be fair, why would anyone genuinely concerned about their privacy choose to join any social networking site?
According to the article, MySpace also enable users to write and share applications/extensions but they are checked and run on the MySpace systems. Anyone who has done application security testing, however, will appreciate that even this process is no guarantee.