Here is another Latin quote for your collection, john39 (I'll leave it for you to translate): non in legendo sed in intelligendo legis consistunt.
And, actually, I am not sure I
have a definitive answer to the question posed. It all depends on the specific facts. It would take a personal review of any website in question and the website's particular terms and conditions to say what can and cannot be done legally about "recapturing" a copyrighted work from a site, or rescinding any rights granted the site, once the copyright holder has voluntarily submitted his copyrighted work for publication on the site.
I
can tell you, xPreatorianx, that a Terms of Service alone will not and cannot transfer your
ownership in a copyrighted work to the site or transfer any
exclusive rights in your copyrighted work to a site.
To transfer the ownership of a copyright in a work, the transfer must be in writing, must be signed by the original copyright owner, and must be an exclusive transfer of ALL rights in the work. With an exclusive license, unlike a transfer of ownership, the licensee becomes "owner" only of the rights in the work that have been granted by the copyright owner under the license (ie, the exclusive right to distribute the work, the exclusive right to make derivatives) and the licensee does not take ownership of the copyright itself. And again, both the transfer of ownership in a copyright and the exclusive license to rights in a copyrighted work must be in writing and signed by the copyright holder to be valid.
It is possible, therefore, for a copyright owner to transfer
some of the rights in a work without transferring copyright ownership. A copyright owner can give permission for others to use the copyrighted work through either the exclusive license mentioned above (which requires it be in writing) or a nonexclusive, or limited, license (limited for a specific purpose or for a particular time, for example), which usually is but does not need to be in writing.
In fact, a copyright owner can license or transfer several different rights to several different people and still retain ownership of the copyright.
If, therefore, you publish one of your works on a site where the Terms of Service state that whatever is posted on the site becomes the property of the site owner, you have effectively given away some rights in your work to the site, but you have not given away any exclusive rights or transferred to the site ownership in your work (both of which must be in writing and signed to be valid).
What you are basically granting a site is the right to use your work - you are granting a license to use your work under the terms outlined in the terms and conditions of the site. The granting of these rights does not prevent YOU from exercising all of the rights you are entitled to as copyright owner - you are just giving the site rights in your work, as well.
Especially when you have a copyrighted work that you wish to protect, it is important to read all terms of service and all conditions of use and all disclaimers posted on any site prior to posting on the site. You do not want to inadvertantly give away rights to your work that you are not willing to give away.
You may wish to have an attorney in your area go over with you the risks you may face in posting your original work on a particular site and what, if any, legal recourse you may have against the site should a problem arise with how your work is being used (or abused). As a note: one of the exclusive rights held by a copyright owner is the right to sue for infringement over the
unauthorized use of the copyrighted work, but the work must be registered prior to any infringement to get full benefit of the protections that are offered under the copyright laws.