At my last (non-library) job, we were actually beginning to implement social media more, so I did have a bit of a chance to work on a social media policy, albeit an unofficial one. Many of those guidelines could translate to a library, as well.
Identify yourself as a library employee. We want to be candid about who is posting from and to our accounts. If you are using a library related social media account or you are using your personal account to comment on our profile or blog, be open about the fact that you are employed by the library.
When posting as a library employee, avoid using the account as a platform for your own agenda. Your personal political, religious, and social views are not necessarily those of the library or its users, therefore it is inappropriate to use the library account to espouse those. Please reserve that kind of posting for your personal, non-library account and use the library account for library-related posting only.
Keep your library posting and personal posting separate. If you do identify yourself as a library employee on your personal account, include a disclaimer stating that you are not posting on behalf of the library and that your views are your own.
Maintain a professional attitude when posting. Be polite when responding to commenters, even if those commenters are not polite to you. Avoid engaging in any online fights. Be respectful of the library users and your coworkers. Remember that you are representing the library.
Don’t disclose any sensitive information. If something isn’t supposed to be public knowledge, do not share it on the library’s accounts or your personal accounts.
Make sure what you post is accurate. Any information you post should be double-checked to ensure that it is factual and/or contains the correct scheduling information if needed.
Use common sense when posting. Remember that nothing on the internet is every truly private, nor can it ever truly be deleted. If you are unsure whether you should post something, check with your superior before doing so.