For open source projects, it’s very popular in the “big data” world. Kafka, as you mentioned, but also Spark, Flink, Summingbird, Scalding, etc.
And yeah, it’s a pretty popular backend language at some large companies. Twitter, LinkedIn, Netflix, the Guardian, Starbucks, AirBnB, Coursera, AT&T, etc. all make significant use of Scala.
Go is certainly bigger, but Scala has plenty of adoption too.