Some employers have a bulletin board that employees can post items of interest like vacation post cards, art and such. Often these are not policed and employers would only take down material if it was inflammatory or in violation of a policy. That being said, most employment attorneys would say that employers should prevent employees from posting personal items up there because once you start its hard to defend making editorial decisions of what is up there. For example if you let employee X post about their side business or school bake sale, its difficult (and maybe an unfair labor practice) to prevent an employee from posting about their own union organizing event.
The central point is that when you let employees post personal items on a bulletin board you need to be prepared to accept material you may not like. In other words be consistent and enforce the rules you establish equally.