It's been used with some internal projects at companies and SOME companies have started building systems with Elixir as a "nicer erlang" nothing major that I know of yet.
The plan is to have v1 happen early this year and after that we'll start seeing more products being built with Elixir.
But really any project using Erlang can and probably will add some Elixir to its code base at some point. The languages complete interop between each other by way of sharing BEAM byte code.
The scale at which Bitcoin exchanges operate at is pretty impressive. Dismissing that due to Mt Gox's poor security is ridiculous. Plus, I don't think the exchanges you're thinking of used Erlang/Elixir.
An industry tends to congeal around a single technology for implementation (for example, Python in bioinformatics), so if bitcoin exchanges will start using Elixir, that is interesting
The plan is to have v1 happen early this year and after that we'll start seeing more products being built with Elixir.