This is incorrect. One of the former VPs said he would not use Scala. Twitter uses Scala massively and has published and continues to maintain several open source Scala projects.
> LinkedIn definitely tried Scala, failed, and went to Java
This is incorrect. A newly hired VP insisted on moving the front end from Scala to Java. In the offline backend stack Scala is used extensively. Check Kafka, Samza from LinkedIn or better yet ask current LinkedIn employees.
Apart from these Spotify, Foursquare, ebay, several banks use Scala. FP is huge in financial institutions, check K/KDB+. Jane Street etc. Thats where most FP programmers work. Also verizon, jet.com and walmart.com. There are several dozen large companies using FP.
FP is mainstream now. Traditionally, it was financial institutions using some flavor of ML/Haskell. With Scala/Clojure on the JVM, Si Valley has moved into FP in a big way.