Public domain
"Pubic domain" is really the wrong phrase. In publishing, "public domain" means the public now owns the rights to the material.
You don't lose any right by placing your writing in front of the public, even the right to sell first rights. Technicall, publishing is not having you worked become public domain, but simply giving the public access to your work. It doesn't matter whether it's printed in a magazine, copied and passed out door to door, left in stacks as a free giveaway to anyone who happens along, or posted on your website. You have made it accessible to the public, so it can technically be considered published by any editor who wishes to make this claim.
But it is NOT in public domain just because it has been published. And any editor who wants to can still buy first rights, or any other rights.
It's solely a matter of whether or not an editor will buy first rights after you've posted work online. You haven't lost first rights, or any other rights, by posting your work. It's simply a matter of editors. Editors are buyers, and you can only sell an editor whatever it is that editor is willing to buy. If he refuses to buy first rights because you've posted your work online, this is neither a legal nor a copyright issue. It's simply the editor's choice, based on whether or not he thinks too many people may have read it. . .and whether or not he thinks he can get the material cheaper by refusing to buy first rights.
Yu can't make editor's buy first right, or any other rights. You can only sell an editor whatever it is he's willing to pay for. By posting your work online, you're simply giving an editors an excuse to pay you less, or to not buy your work at all, because of fears his readers will have already read your work online, and no editor wants material that has already been seen by his own readers.