This is especially important for those staff that explicitly represent the organisation on the internet. If the organisation happens to handle sensitive information then knowing what staff can and can't say is even more important. This problem is compounded for organisations such as government departments, hospitals, and local councils who are bound by legislation that prohibits the release of key internal information, documents, decisions, etc.
Because people increasingly use social media in their everyday lives, this means there is a risk that privileged information might get inadvertently leaked; in addition to the usual undesirable behaviours such as bitching about the boss, harassing other employees, etc. Consequently, providing some clarity around what is expected from staff - as social citizens as well as employees - is now a necessity.
Fortunately, guidelines do not have to be onerous or draconian to be effective. They need to be expressed in a way that staff will acknowledge their existence and likely act on them.
For example, try using language that is positive and active rather than negative and prohibitive (see the University of Otago internet usage policy as an example of the latter). Moreover, if staff are involved in the creation of these guidelines then the likelihood of staff buy-in is improved.
The Victorian Department of Justice in Australia have put together a comprehensive and pretty robust policy. It is clear they have put thought and resource into producing these guidelines - as evidenced by the video below.
The intention of the policy is to "establish a culture of openness, trust and integrity in activities around Web 2.0". This is of course laudable and even though this policy is very long and has some bureaucratic language still, it is on the whole a very good piece of work.
Take a look at the video below (ignore the claim that email is social media as that is plainly erroneous) and if you feel compelled, skim through the Department of Justice policy. You might just find a framework that works for your organisation.