What part of the industry are you in where flatbuffers is seen as the de facto standard? Personally I've never randomly encountered a project using flatbuffers. I see protobuf all the time.
(I've randomly run into Cap'n Proto maybe 2-3 times but to be fair I'm probably more likely to notice that.)
Flatbuffers seems to have penetration in the games industry. And it sounds like from other posters that Facebook uses it.
I recently started a job doing work on autonomy systems that run in tractors, and was surprised to see we use it (flatbuffers) in the messaging layer (in both C++ and Rust)
As of the last time I was close, flatbuffer usage is or was near ubiquitous for use in FB's (ha ha, go figure) mobile apps, across Android and iOS at least.
It has established itself as the de-facto standard, with a few other places using SBE instead.
In any case the main problems with binary serialization are:
- schemas and message version management
- delta-encoding
If you ignore these, flat binary serialization is trivial.
No library provides a good solution that covers the two points above.