John Jermain Memorial Library has provided a DRAFT copy of their Social Media Policy.
Social Media Policy Draft John Jermain Memorial Library
Eric Cohen would appreciate and comments to help finalize the policy.
Another example is the Whitman Public Library of Massachusetts whose policy has been cited in the following article:
http://www.schoollibraryjournal.com/article/CA6699104.html (though the link is now bad).
Social Networking Policy Whitman Public Library
Also try this social media policy toolkit: