LinkedIn uses it for their mobile backend.
But I more than suspect that the mobile backend is just a simple frontend to the real backend. And that is written mainly in Scala.
As far as I know the closest thing to node.js in Scala would be something like Finagle, which they use at twitter:
For my current hobby project the backend is written in Scala, Akka and Play! - though I am considering replacing Play with Spray, since all the backed does is serving JSON from REST services.