I have been working on the same topic. I found a lot of social media policies online, just by doing a google search. Some were from banks and some were from other industries, but all had similar topics mentioned.
Here are some basics to include:
1) Define social media.
2) Let employees know that you could monitor their use of social media and they should have no expectation of privacy.
3) Social media use should not interefere with performance of job duties.
4) Employees must identify all opinions expressed as their own, and comply with all applicable laws.
5) Employee must not communicate any private information regarding the bank, co-workers, customers, etc..
6) Specify who can use social media and what subject matter can and cannot be posted.
Also beware of posting anything that will trigger advertising requirements. (Perhaps a department could review all social media posts)
Hope that helps!