Image by Scott McLeod via FlickrMembers of the KIN 'Enabling Technologies' Special Interest Group have been aware for some time of the futility of trying to 'manage' social media by edict or restricting access to web apps. Instead, members have shared their good practice in clearly and unambiguously setting out their organisation's policy and what is acceptable practice. There are two very good resources that I'd like to point you to:
Carl Haggerty's blog has links to sound policy and practice for Social Media. This is aimed at UK Local Authorities, but I defy anyone to say this is not applicable to any large organisation.
The second is from the always excellent Commoncraft on the topic of Protecting Reputations Online