It is not that tricky to document a fairly clear policy. What is damn difficult is communicating it effectively to employees.
Also this reporting about cases in the US supporting freedom of expression relating to workplace conditions, pay etc does not help much.
In SA we have different laws and even with our liberal LRA you can't trash your company or boss on Facebook.
You can get fired and the CCMA and the Labour Courts will support it(depends on what gets written). If your words, even if they are true undermines workplace relations, damages reputation etc. then you can get sacked.