Organizations need to have a conversation about social media and its implications. Social media doesn't usually create new behaviors. It amplifies existing ones. If you have policies governing the following, you may not need a social media policy

• Confidentiality of information
• Appropriate Behavior - Interpersonal Conflict, Harassment
• Who acts as a spokesperson on behalf of the organization
• Protection of personal information
• Internet Use Policy
• Approvals on Web communications
• Use of logo or representation of brand by outside stakeholders
• Intellectual property rights and other legal issues

Some other questions to ask:

• If the lines between your organizational/personal presence are squishy,  what is appropriate conduct? 
• Does your organization need something more formal than "using common sense?
•  As the organization's leader and if you aren't comfortable or have first-hand experience with social media, how do you trust your staff?

Consider the following when crafting an acceptable use policy:

• Build on your existing public communications/media policy
• Look at examples from other similar organizations (Online Database of Social Media Policies)
• Consider whether the policy applies to volunteers as well as paid staff