There are people discussing your industry, your sector, your competition or (specifically) even your business. These ubiquitous platforms of communications are instant and, as they say, a negative comment not properly addressed could do as much harm as good.
Think about this: In addition to people talking about your company and its services/products, policies, your employees are quite possibly using social media in some form or another on a personal level. And what are the odds that some of their 'friends' are people that they have befriended in the course of business or social life. More than likely, that 'friend' knows your employee as a representative of the company (no matter how friendly they are). And who’s to say that they may not take offence to some comment that is made on social media.
So just avoiding social media during working hours is no longer reasonable. But, like any other means of communication, you need to have a policy in place.
I found this article from Fast Company on the do’s and don’ts of setting up corporate social media policies. There's some great advice from companies like Intel and CNN that I think we can benefit from. My highlights are the ones below that should stand any company in good stead:
Stick to your area of expertise and provide unique, individual perspectives on what's going on at your company and in the world related to your business sector
Avoid politics and religion because these can be misconstrued quite easily and blow up in your face
Post meaningful, respectful comments in response to social media posts —in other words, no spam and no remarks that are off-topic or offensive (the best rule is if you don’t have something nice to say, don’t say anything)
When disagreeing with others' opinions, keep it appropriate and polite
If you do disagree with a post – send private messages to resolve the disagreement before resorting to a public forum
Always pause and think before posting. That said, reply to comments in a timely manner, when a response is appropriate
Respect proprietary information and content, and confidentiality