F# is used in quite a number of firms, especially in the Chicago area I am aware of a handful of companies. Initially it was used just for prototyping/modeling type stuff, but more and more it is seeing the light of day in production. It will never see the adoption scale of Java/C# but that isn't the point or the goal.

It makes sense as well if you follow the history of most of these trading firms. Most of them started out just manipulating excel spreadsheets. Then brought in vb macros and such and then from there some diverge into java or go straight to some functional programming solution like ocaml with jane street, but others stay tied to msft with C#. Then F# comes along and allows intelligent developers at these msft firms to express themselves more clearly when dealing with the type of math problems they are solving.

If the firm is using C# I am almost certain that F# is in the mix there as well.