Jane Street is a serious functional programming shop. We use OCaml, a statically typed functional language for almost everything and have what is probably the largest OCaml codebase anywhere.
This leads lots of people to think that they shouldn't even bother applying, under the assumption that we are only interested in hiring deep functional programming gurus. I think people get to this conclusion in part because they think of functional languages, especially those with fancy type systems, as arcane tools that can only be used effectively after deep study.
To the contrary, one of the reasons we started building production systems with OCaml was that it was relatively easy to understand, even for people with no formal CS background. Since then, we've had good experiences taking students with no functional experience at all and getting them to the point of being able to complete a project in just a few weeks. We also have a very successful "OCaml Bootcamp" program, where over four weeks, we train all of the incoming traders and many other non-developer hires on OCaml and our development tools and libraries. By the end, most of them are able to create useful applications.
All of this is to say that we don't go out of our way to hire people who are already familiar with functional programming. In practice, it's just not that hard for strong programmers to pick it up after they start.
That said, a solid majority of the developers we hire do come in with functional programming experience --- but that's because of their preferences, not ours. Programmers with an interest in functional languages have an extra reason to want to work here, and so we get an unusually high number of good applicants from that pool.
There's a more general lesson here: using well-loved tools is a good way of attracting (and retaining) great developers.