I just wonder where do functional programming languages are used? Might be some good examples? Like rocket science or telecommunications?
Well, here's a list of projects using Erlang:
The world doesn't revolve around building apps for tablets and mobile phones ;)
Usually functional languages are used when there is a need for a very high degree of abstraction, parallelism and scalability. This doesn't mean you cannot write code with these properties using any other language (non-functional), but the absence of side-effects on a purely functional language helps in these scenarios.
Erlang is a great example of this, since it was built from scratch to address problems that were "embarrassingly parelizable".
There is also Scala, that lets you program in a mix between functional (the preferred way), and procedural style. Scala is used, for instance, on the Twitter middleware layer to handle the absurd level of concurrency that they deal with.