Flatbuffers targets C++ and Go[1], so it does not appear to require JIT. It would appear the necessary code is generated statically even for Java and C#, to take advantage of strong typing.
Makes sense but do people use it for conventional web. If there is such a great deal of efficiency why isn't there a great deal of adoption?
Edit: Added some more text
Because JSON is the "default" (everybody knows it, everything has tooling for it) and most people don't care about efficiency on that level until they have to.