If so, have you found and read their “terms of service.”
Not the helpful FAQs they might have, but the down and dirty TOS in all its legal glory. One writing site I that was recently recommended to me as a “safe” place to post my work says this:
However, with respect to Content you submit or make available for via the Service, you grant Aresta Enterprise a perpetual, irrevocable, world-wide, royalty free and non-exclusive license to use, distribute, reproduce, modify and publicly display such Content on the Service.
Now, on its help section this site tells writers that the site doesn’t get paid or have exclusive rights to the story. True. But the TOS says writers don’t get paid and writers no longer have exclusive rights to the story.
When you sign a contract you generally sign a statement that you currently retain all rights to your material and therefore can assign rights to the publisher. But according to those Terms of Service, this writing site also has rights to your work and can reproduce that story and display it for free, without paying you royalties.
Some sites have terms like this that also include a section that states when you remove the content from the site, all the rights claimed above no longer apply. Which means you can post content, have it reviewed and remove it so all rights return to you. But in this case, that clause is missing.
Here is what Facebook says:
For content that is covered by intellectual property rights, like photos and videos (“IP content”), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (“IP License”). This IP License ends when you delete your IP content or your account (except to the extent your content has been shared with others, and they have not deleted it).
Have you read the terms of service where you post your work?