Timotheus,
Thanks for letting me know about the discussion, and no, I've never heard of such a thing. Here are my thoughts as to why I don't think this is an issue:
[a] The person who writes something is the copyright holder according to law. Just because it is "hosted" on a particular site does not give them the right to use it for profit. You have only given them the right to display it exactly as you have typed it in. No more ... not less. You are the ultimate owner of your work.
[b] A few snippets from your book is not something that Facebook could or would attempt to capitalize on ... would anyone really pay to read such snippets when it is available for free on your Facebook page? The very act of giving the snippet away for free makes Facebook's monetary interest in the work worthless.
[c] Samples you give are not the whole book. No one, not even another author could duplicate the rest of the book and sell it. It would be something completely different and not your book. Even then you could sue them.
[d] If Facebook attempted to do something like that the bad press would NEVER offset any money they could make. They are well aware of their reputation and keeping up a good image. Thus they will never do something like that.
[e] One of Marcher Lord Press's authors is publishing an entire book on Twitter right now and then when it is done will make print copies available. This would not be possible if such things existed.
But ... keep in mind that a traditional publisher (if the WHOLE book were available for free online) might balk at taking the book on ... in that case they are concerned about first publishing rights. Just give a few tantalizing snippets and you are fine.
If you are concerned you could always put a © at the bottom.
Also, all of my blog posts automatically feed Facebook for me ... and that is what I want. There is really no difference to me.
This is my take.
-Robert