I've worked with people internally, and it is indeed a challenge to speed up services with FlatBuffers. This is because 99.9% of internal communications flow over Protobuf, and many such services are simply: receive Protobuf, modify a few things, send it on the next service. So to deploy FlatBuffers there, you usually have to translate from Protobuf to FlatBuffers, do some high intensity stretch of communication in FlatBuffers only, then convert back. Given the cost of conversions, it would require replacing fairly large amounts of infrastructure for it to be a clear win, and given that those conversions have to be written entirely by hand, that is often not feasible. So translation of what the other comment said: "we gave up".

FlatBuffers is used to great effect, but often with new teams building new things, that are not yet shackled in Protobuf everywhere, like many of the AI related teams.