Interesting - I didn't even know that there was a monolith at Facebook [ed: beyond there being reports that they use a large monorepo for code]. I stand corrected, thank you.
This doesn't directly address if graphql at Facebook federated different data sources (eg graph and sql databases) - but I suppose not?
I don't have any "insider" information regarding the data sources used by the Facebook monolith, written in PHP, when GraphQL has been developed, but I would expect it to talk to multiple/different datastores, and not just MySQL. Then yes, it could be argued that it has been used to federate multiple data stores, even though it was already the case before using GraphQL :)
This doesn't directly address if graphql at Facebook federated different data sources (eg graph and sql databases) - but I suppose not?