Well, I have all 3 OAuth's working on my development FMS 16 server happily and I can log in from all of them. Pretty nifty.
One comment though. If I am logged in on a web browser to Google or Microsoft and I click their logins below in FileMaker, it is a single step to get logged into FileMaker (single click). If I use Amazon, it takes me to their web page and I have to click a button in their web page to sign in. So, kudos to Google and Amazon for making it a simpler process even though I'm still pretty happy with Amazon.