Generally speaking, you can prohibit your employees from engaging in certain activities (including social media use) during work hours or while using your equipment and/or communication systems—and discipline them for doing so.
From a practical standpoint, the best way to manage the risks associated with employee social media use is to develop a comprehensive policy that is distributed and signed by employees when they are first hired. The following guidelines can help you draft your social media policies, which should be applied and enforced in a fair and uniform way.