I don't remember entering into any sort of contract or license agreement with HN (besides the obvious that HN can make zillions of copies of my work to show them to other users). Users have copyright to their posts and comments as expected.

I don’t remember entering into one with Reddit

FB, I think it was sort of implied.

We all did when we signed up, or if I suppose. Just not the recent ridiculous data training with no optout changes.