Facebook, WhatsApp & Co. at the workplace

We are already familiar with this subject: there, the Facebook profile is renewed and someone is chatting with friends, instead of doing the job for which they are paid. In this regard, the labor-law related problems do not substantially differ from the issue of private internet use at work. However, I would like to look at the use of Social Media from a totally different perspective. How can I react as an employer if an employee comments on me or my company on social networks in a negative way? And how do I cope with the fact that an employee is spreading opinions on the web, which I hardly...