GraphQL is not used for service to service communication at Facebook. It was originally built to layer over our application logic just before data is sent to mobile clients, and has been used that way ever since.
Most of our service to service communication happens using Thrift (https://github.com/facebook/fbthrift) originally built by Mark Slee at Facebook.
There are some commonalities between GraphQL and Thrift, they both define a type schema and they're both handling RPC. However they then diverge. Thrift works well with a speedy service to service connection (usually within a datacenter), and expects tons of requests per second from a single client, while GraphQL is designed for crappy slow mobile connections, and maybe a few requests per minute per client. Thrift instead focuses on extreme efficiency at the network and memory levels, and is often used with services written in C++.