Kindle Users
Kindle users need not apply right now. Facebook seem to have been singularly incapable of providing the messenger app in the Amazon appstore, so if that version also goes messengerless, they'll hit a bit of a snag.
Granted, it;s possible (although fiddly and not always successful) to add the google play store as well, but if they can publish the main app on the Amazon store,the messenger shouldn;t be too much of an issue.