Posts on Stack Exchange sites have a Creative Commons license applied. However, on Writers, people will post snippets of their own writing for critique purposes. Does that mean that if I post a critique question, it CC's the selection of work I post?
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Just to add to Fabian's correct answer: as he says, you remain the owner of the snippet, but by posting on a SE site, you grant the community the irrevocable right to use the snippet under the CC-Wiki license.
Note however that anybody re-using CC-Wiki content has to explicitly attribute both Stack Overflow and the author of the snippet. That greatly reduces the snippets' usefulness to copycats. I'm not a lawyer and know little about IP, but I can't see posting a snippet on writing.SE posing a problem for the author (except that they maybe should inform any future publishers that parts of the work are published under a CC license - if only to avoid inadvertently violating the "no previous publication" clause you mention.)
I created a scientific infographic on the issue once and added some more specific information:
However, the situation where you put something onto writing.SE, people suggest corrections, and you use them in your work, is different. Technically, if you copy & paste another user's work, that demands attribution no matter how small the snippet is. The only way to avoid that is for the user to grant you the permission to use it without attribution, which they can because they're the content's owner. I have no idea how to do that in a legally binding way though, you'd have to ask a lawyer.
This is documented in the Terms of Service in Section 3: Subscriber Content
Now, you're asking two different questions here, so to clarify:
Additionally, all answers, revisions, and suggestions posted in response to your question are also copyright by their respective authors, and licensed for use with attribution.
Everything you post will be licensed under the Creative Commons license, that doesn't mean you give up your copyright (which you're not even able to in some jurisdictions). It means that others can use those snippets you post under the terms of the CC license, but you still retain ownership of that content. You can of course release that snippet under a different license later, though that won't stop people from using it under the CC license.
What you need to be absolutely careful with is posting content you don't own here.
I'm not a laywer so don't rely on this post for anything important.