Correct me if I'm wrong, but it sounds to me that magazine publishers are much more concerned with this (first publication rights) than book publishers currently are.
Every site I've seen telling writers that they've given up "first publication rights" by posting on the Net (and that's usually how they word it, Miss Snark) refers back to magazine and literary journal submission guidelines. I don't know if that means book publishers just haven't jumped to the conclusion that "on the web" = "published" yet, or if it's simply a difference between types of publications.
Book publishers buy the rights to publish in various forms, and languages, and geographies.
First North American rights is for a book sold in the geography of North America.
First serial are rights for magazines to print pieces of the book before publication of the book.
Electronic rights are the rights to publish in ebook form.
Notice there is no "first" in front of e-rights.
Book publishers mostly don't care if it's been on the web, as far as I can tell. They care if it's been in book form, or pieces of it have appeared in print in magazines.
Magazine editors who are competeing directly with the web care a lot more.
There are lots of reasons not to put your novel on the web but losing "first publication rights" for a book isn't even on the list.
as to the giant squid: pieces of giant squid wash up as debris so people talk about them all the time but no one has, as far as I know, ever seen one alive and zipping around in the darkest depths of the sea.