that message is not even in the ,mobi book file, it's in the Kindle firmware & cannot be turned off.
it is triggered whenever you turn the final "page" of any document, even ones you've self-created.
It's one of the reasons why I switched from Kindle to Sony , I ranted about it on Kindle forum but got little sympathy....
as I said there, the twin assumptions that the whole world needs to know or care that I've reached the end of a book AND that everyone who reads books is hooked on social media inanities like twitter is [ insert adjective of you choice ]
expect more Kindle advertising to come iwith future updates....