You own the copyright to all of the (original) writing you post here. Let me rephrase: anything you post here (be it stories, messages, comments, etc) do not become our property.
By posting your material on this website, you licence it to us so that we have the legal right to share it with the rest of the Internet. We do not own it, but are allowed to use it. How is this different? If we owned it, you would hold no rights to it; however, since you are the owner, you retain all rights and can do whatever you want with your work. We could not stop you if you wanted to publish the same work you posted here in a book, nor would we be entitled to any royalties from that.
To ensure that it is "on record" that you own the copyright, you can register your work with the Library of Congress. A bit overkill if you'd ask me, but that's how people do it.
It is your responsibility to ensure that your work stays safe. The database holding your writing (and a lot of other stuff) is backed up on a nightly basis to a server on the other side of the country. However, anything could happen: I could get hit by a bus, there could be a nuclear holocaust, etc. Although there is no mechanism to export your storygames to a single file, you could always print off, save to Word, or otherwise save pages from your stories.
