Social Media is accessible everywhere. It is no longer a second sister to mainstream media. Your employees will post to their social channels with or without your guidelines. So my suggestion is to draw them up now, and share them with your employees before it’s too late.
It is important for organizations and companies to keep track of what their members and employees are tweeting and posting about especially on issues that pertain to the workplace. Will you be hampering their rights to freely post? Should you just ban them from accessing their social media sites?