Copyrights are a tricky thing. As far as I know,
Fair use applies to all things posted upon the internet, at least in regards to forums. The copyright is still yours, as it is presumably your original work, but by posting it upon forums you are indeed allowing your copyrighted material to be viewed by the general public. They can't copy it and claim it to be their own, by any means, but you still are agreeing to let them see it and/or use it by posting.
That said, I don't believe you can "get" a copyright, as it is inherently yours to begin with. If you are hesitant about posting something, then don't. Sometimes publishers will be wary of a work that is already out in the public, as then why would anyone buy it if they can get it for free? Use discretion and your best judgment.
I'm no law professor, so I could be wrong, but that's at least the way I perceive it.