Sophos released an advisory on security risks with social networking this past week.  ReadWriteWeb has a great article on the subject, and has this recommendation:

The real solution to the social networking security problem is to embrace sites instead of blocking them. Rather than being overly restrictive, companies would do better to make social networking policies a part of their corporate policy and procedure manuals, spelling out what is and is not acceptable within their organization. Left up to users, it’s clear that for some, it’s anything goes…and that’s a danger most companies cannot afford.