I write social media policies. But whenever I'm asked to do so I review the relevant organisation's culture, market and personnel. As we'll see this context is important if you want to know how to write a social media policy.
A social media policy can be as short as a few paragraphs, a few pages or longer. A consideration governing length is - what is the purpose of the social media policy? In simple terms there are two purposes usually present - messages on what conduct is approved and what is disapproved.
As social media continues to grow (see graphics below), so do legal issues. People are people, offline and online.
Social media is still "new" (particularly for audiences familiar only with television, radio and newspapers). This keeps it ripe for moral panics and exaggerated over-emphasis in traditional media. Their focus is on bad things that can and do sometimes take place with a connection to Facebook, Twitter or other social media sites.