From the Ember Community Survey, a large number of people who use ember are using JSON API in production, so it’s much more than that thread. We haven’t done a good job of collecting them up on a page, though. We should.
I have an API that follows the spec more or less, with minor differences detailed in the docs such as nested resource creation (discussed more in #1089). I’d also be interested in seeing a list of other APIs, especially to see how they’re writing their language bindings.