Intel has published their Social Media Guidelines.
I like their approach of giving users guidelines for they should and should not do. The context is to place the responsibility on the individual. It is that person who is creating the content. They are responsible for the content and the consequences. I think it is a great balance of encouraging people to interact with responsible behavior.
These guidelines are actually a great starting point for a law firm. You just need to add in a section about not creating attorney client relationships and a section about attorney advertising disclaimers.
Intel could have added some specific recommendations for some high profile sites. Of course the sites are changing so often that it might be hard to keep the policy up-to-date.