Same as React: do not use if you plan to do anything that Facebook might dislike or steal, or want to sell your startup, or work in a bigger corp (ask the legal guys before).
Of course, you do need to ask yourself these legal questions if you use Relay.
Apart from that GraphQL is the only one library with a known patent by Facebook, so you would actually want to get the patent grant there.
[0]: https://github.com/apollographql/react-apollo/blob/master/pa...
[1]: https://github.com/apollographql/apollo-server/blob/master/p...
[2]: https://github.com/apollographql/apollo-client/blob/master/p...