Social media is all about listening and contributing where you add value. 'Two ears, one mouse' is one way of thinking about it....
Here are some extracts from bigger companies social media policies:
Don't pick fights, be the first to correct your own mistakes, and don't alter previous posts without indicating that you have done so.
Try to add value. Provide worthwhile information and perspective. IBM's brand is best represented by its people and what you publish may reflect on IBM's brand.
With conversations, participate online. Don’t “broadcast” messages to users.
Perception is reality. In online social networks, the lines between public and private, personal and professional are blurred. Just by identifying yourself as an Intel employee, you are creating perceptions about your expertise and about Intel by our shareholders, customers, and the general public-and perceptions about you by your colleagues and managers. Do us all proud. Be sure that all content associated with you is consistent with your work and with Intel's values and professional standards.
Be real and use your best judgement.
Here's a great article on 10 guidelines to follow: Econsultancy’s 10 social media guidelines