Should a small business have a social media policy in their handbook? Absolutely. You need to be careful what you say, what you tell the employees they can and cannot do. But if the employee is representing their employer, absolutely there should be repercussions. If the employee says derogatory things about the employer there should be repercussions, definitely.
There are labor laws that will not permit employers to reprimand an employee if they’re talking about salaries and things like that.