system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a company fire an employee for a Facebook comment, but not sue a non-employee for defamation because of a Facebook comment?
gpt
The standards are different. 

In the overwhelming majority of cases, an employee is "at will." This means that the employer can fire them for any reason or for no reason at all. In the present day there are some restrictions on this---you can't fire someone because of their religion, for instance---but very few. 

Thus, posting something on facebook can get you fired, but so can not posting something, or posting the wrong thing, or any other thing that the employer doesn't like. 

A lawsuit for defamation is very different.  Here, you're not dealing with the employer's decision to keep someone on the business's payroll, but with the employer's desire to use the courts to get something.  In the U.S. at least, the standards for defamation are very, very hard to meet. As such, very few of these cases are filed. 

As a note, though, this is not any different than suing an employee. You can fire the employee for their facebook post, but unless you met that same high burden, it would not be any easier to sue the employee for defamation than it would be to sue the third party.