Most organisations have technology use policies setting out the authorised use by employees of work computers and smartphones as well as their use of the internet, email and blogging. These policies need to be reviewed to cover Facebook, Twitter and other social media and when a breach of these policies can lead to termination of employment.
The principles are the same: do not do anything in your personal capacity that could affect your work life or could be likely to bring the organisation into disrepute (eg by breaching laws relating to discrimination, harassment, obscenity, defamation and breach of privacy) and do not disclose confidential information obtained through work. Be respectful. Remember, the internet does not forget!
Examples of social media policies can be found at Compliance Building’s Social Media Policies Database and Social Media Governance.