Hmmm I'm not a native speaker, although I learned English at a very young age and now regard it as my mother tongue - in the sense that I think in English and I speak it more freely than Romanian.
I've always written a great deal, even before Steemit, so I don't think it is the platform's merit, if you get what I am saying. But yeah, definitely, writing a lot really improved my English. It's helped me become really familiar with the language, sort of like with an old friend. And since I kind of picked it up (the language, as opposed to studying it), I've always written as if I was telling a story or talking to someone, you know. So yeah, writing and especially communicating with others in English helps a lot.
I've even met some Romanians online who asked that we communicate in English, to improve their skills.