The mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior.
Like the Façade pattern, I can absolutely see the logic of wanting to use a mediator. It is supposed to make it easier to work with a set of objects, because it hides their interactions.
In practice, almost all known cases are bad ones. In fact, in most systems that I have seen any association of the name to the actual pattern it is supposed to represent is not very associated at all.
The differences between façade and mediator are minute, and you would think the same advice would apply. However, while you can find a lot of usages of facades (or at least things people would call facades), there are very few real world examples of mediator pattern in use. And almost all of them carry the marks that say: “Just read GoF book, @w$0m3!!!”