Jason - I like your house rules suggestion.
In my somewhat limited experience, these rules of engagement have to give employee a sense that they are trusted and respected. I've seen too many situations where a social media policy (mainly written by the legal council) mutes the voices of potential evangelists.
So, a short handbook might include these chapters:
- Chapter one - Why we need each other
- Chapter two - Why we trust you
- Chapter three - How we use social media
- Chapter four - How we'll change the world