Since early 2013, we at IMVU have used Haskell to build several of the REST APIs that power our service.
When the company started, we chose PHP as our application server language, in part, because the founders expected the website to only be a small part of the business! IMVU was primarily about a downloadable 3D client. We needed “a website or something” to give users a place to download our client from, but didn’t expect it would have to be much more than that. This shows that predicting the future is hard.
Years later, we have quite a lot of customers, and we primarily use PHP to serve them. We’re big enough that we run multiple subteams on separate initiatives at the same time. Performance is becoming important to us not just because it matters to our customers, but because it can easily make the difference between buying 4 servers and buying 40 servers to support some new feature.
So, early in 2012, we found ourselves ready to look for an alternative that would help us be more rigorous. In particular, we were ready for the idea that sacrificing a tiny bit of short term, straight-line time to market might actually speed us up in the long run.
http://engineering.imvu.com/2014/03/24/what-its-like-to-use-haskell/