I ran into a TechNewsWorld report on people being stupid on social networks. Let's look at it from your company's angle - could an employee tarnish your image on social media? The short answer is yes. The long one is that it's you who needs to be smart.
I'm never a fan of big brother approaches. Therefore I preach to accept and embrace the fact that your employees are networking, rather than closing your eyes to it. By making them aware that they are out there representing you, you could even find they become your advocates! Furthermore, by going out and making sure that you represent yourself you'll also be part of the conversation - better than hearing about it in tomorrow's paper.
As Southwest’s Herb Kelleher put it - “The way you treat your employees is the way your employees treat your customers.”