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.

I assumed it would be on an instance by instance basis.

I think that we should just assume at this point that if you post anything online, it will be used by other people with or without your consent.

Therefore if you want to protect your creations (whether they be text or other), you should obtain copyright for and host your content yourself or through a privately-owned service that protects your IP, then share a link or embed that content into social media. I'm not a lawyer, but I expect that this is sort-of how it would need to be. While there are some international agreements like the Berne convention that protects IP from the moment it is created, I do believe that it can both be waived by the creator in a ToS, and in order for you to pursue legal action (in the US at-least), you often need to go through the complete process of obtaining a copyright, trademark, patent, etc from the government.

While I love the ideology of copyleft and I am a socialist at heart, if you live in a capitalist economy and want to earn a living, there are compromises you have to make.

If it works anything like USSR, then you as a user don't own anything and have no rights to anything. Same as Reddit pretty much.