First, Dassault Systèmes doesn't fall "under one umbrella" with Dassault aviation, it is an independent public company (with the Dassault group having minority shares in the company). And just because a software company has defense related products doesn't make it a “defense company”, nobody would ever call either IBM or Microsoft “Defense Companies”.
Then Facebook doesn't seem to be using Ocaml in Facebook Messenger, and instead uses its standard tech stack for that (the Hack PHP derivative).
The fact that Facebook has a developer-facing team building internal development tools with OCaml doesn't suffice to change the status of a language: since there are barely any Ocaml developers at Facebook, an Ocaml enthusiast would have to be very, very lucky to land an Ocaml job there…
And that's the whole point of the “hobby language” status: can you land a job using it or not, and Ocaml simply isn't that kind of language. And again, France has been teaching Ocaml to at least a hundred thousands of students over the years.
Dassault Aviation makes the planes themselves. All Dassault companies are under one umbrella anyway.
The OCaml website lists a bunch of other users.
In my opinion, none of that really matters, any single one of those uses proves it's utility... Facebook Messenger alone is probably used by more people than every piece of Lua software ever made though.