Stackoverflow, and the rest of the SE network, explicitly says that all user-generated content is licensed under CC-BY-SA. (link here). So, while SE has the right to do whatever they want with user content, they have to attribute the users who made it, and they have to keep the same or similar open license on the content. I know users can't really fight a big company on equal footing, but an explicit license like that is an implicit commitment to respecting, at least to some degree, users' ownership of their content.
On the other hand, Reddit's user agreement includes this: "...you irrevocably waive any claims and assertions of moral rights or attribution with respect to Your Content...." So, reddit asserts the right to use user content however it likes, with no rights to the users who generate it.
Recent events make me much more interested in knowing how the content I generate will be licensed. I know a cc license on Reddit content wouldn't change most of what makes the recent decisions so terrible, but it would give some standing to the people upset with how reddit plans to use what they've contributed.
I looked a bit, but didn't see an explicit statement about how the content in this server (lemmy.world) is licensed. (That's not a criticism; I think the admins have been busy with a few other things, and I really appreciate it!! I'm asking about this because I'm hoping to see more and more here.)
The creators are believers in copyleft, so I'd assume your stuff becomes public domain.
Copyleft is nothing like public domain. Licenses like the GPL would be considered copyleft, where the license itself is required to be maintained when distributing the work, allowing access to the sources that made the work.
It's also something that would have to be explicitly stated in the Terms of Service or something similar.