Medium = blog posts
Hacker News = blog posts by other people related to all things tech
Hashnode = Cross between Facebook / Medium / Stackoverflow and any other IT related subjects (serverfault comes to mind)
Though I posted a "story" once speculating on happenings at Apple and it was deleted as off-topic so maybe not so much FB as SO (ie: moderated)
I don't consider it to be a place people find me on - for me, at least, that's LinkedIn.
(I feel I can speak to this) Any community eventually expands outside of it's initial scope based on how users use the system and what features they want. Failure to do so could mean failure of the idea / system. IE: if your (the site) not growing with your users, they'll find somewhere that does. Hacker News is the exception to this (of those listed) as I think they just don't care, haha - I've been on hacker news for years and they only ever make minor changes. Not that it needs a lot, but it could stand for a major feature improvement.