Erlang has been used by Ericsson for many years in their telecomm software, and other companies are using Erlang as well. Amongst those most prominent are:
* Amazon, who is writing their SimpleDB in Erlang
* IMDB (Internet Movie Database) is switching from Perl to Erlang
* Facebook has implemented their chat software in Erlang

F# is a relatively new FPL, but already software companies are adopting it. And Microsoft is making it part of the official .NET language stack soon. Besides, F# is “just” OCaml with bi-directional .NET interoperability and a performance profile comparable to that if C#.

Haskell is also used in several professional applications, and other FPL’s have their niches too.

I am not familiar with the Scala programming language, except that it runs on the JVM, integrates with existing Java libraries and supports anonymous functions, higher-order functions, currying, pattern-matching and other important FPL features. That makes it somewhat equal to F#, but is the community as large, and is the support (from both developers and companies alike) as that of F#? I don’t know. But given the above mentioned “features” of Scala it should be worth considering as on of our tools, definitely.

The landscape of programming is rapidly changing these years, with numerous functional and dynamical languages emerging. Some will come out on top in the end (if one can speculate about such a concept), but most will stay in the shadows. It all depends on features, community, integration and how much the language will evolve (like C# which has evolved quite a bit… perhaps too much).