Functional languages may not be directly useful for building great apps but we use functional programming paradigm heavily for building our Apps. Pure functional programming puts the constraint of "no side effects". This ensures that pure functional calls will yield same result in what ever order they are called. This is not ideal for web-development but if functional programming is combined with a state-changing system, a robust web app can be built. Have a look at my paper for more details: FAST Server
Also these slides.
