I guess I don't understand. How does one go about writing an open API for someone else's platform? Won't that require support by that someone else who created

I guess I don't understand. How does one go about writing an open API for someone else's platform? Won't that require support by that someone else who created the platform? Otherwise, aren't we stuck with having to recreate Facebook, Twitter, etc. in order to write an open API for it? And therefore stuck requiring the adoption for the new platform that someone else's platform already has? Therefore "rolling your own" entirely in the first place?