We use them internally at Square for our RPC mechanism ("Sake", similar to "Stubby", Google's internal RPC mechanism), for our Kafka-based logging/metrics/queue

We use them internally at Square for our RPC mechanism ("Sake", similar to "Stubby", Google's internal RPC mechanism), for our Kafka-based logging/metrics/queue infrastructure, and for defining external JSON APIs. We're in the process of switching from Sake to GRPC, which also use Protobufs as their payload format (although you can sub in different transports).