To answer your question - Yes, it can certainly be used in production. Lee Byron, in his recent AMA on Hashnode, said :
Relay is an implementation of the Flux concepts. Relay requires you to have a GraphQL service.
Our iOS and Android apps are almost entirely powered by GraphQL (but not Relay, as they're not JavaScript).
Parts of those apps, and in some cases whole apps (Ads Manager) are built with React Native and use Relay with GraphQL.
Relay and GraphQL are being used inside Facebook. So, if it suits your needs go ahead and use it in production.