For facebook and big corporations, you usually agree to the ToS/EULA before you actively using their services. The clause there usually protects their ass by stating you give them the license to basically do whatever the fuck they want. Sometimes even giving up the copyright entirely, like some CLA when contributing to open source projects.

But lemmy, as far as I remember, don't have such term. So it is an interesting question since if the instance doesn't impose a legal requirement for you to give the instance a license to do anything besides storing and serving it verbatim (like many other user-content sites. deviantart comes to mind since the user can license their image iirc). And yes, words or a string of words can be copyrighted and licensed because we do have protection for books and other text material.