This is a step-by-step guide for those you try to implement FB Login in 2019. I lost pretty much time on figuring it out, so it would be cool if my recommendations help someone to save it.
I assume that you have already installed your FB plugin. First, in order to get APP ID/ API Key and Secret Key, you should go to https://developers.facebook.com/ and create a new app.
Second, you should enter your full website address as a site URL. Note, it is important to add https:// before it as it is done below. I entered the same full address as App Domain at the screenshot above.
Third, go to FB Plugin Page on Bubble. You can find a generic redirect URL there. This is an example : https://yourwebsite.com/api/1.1/ oauth_redirect
Fourth, take it and paste on Facebook for Developers page under the section FB Login (Settings) as a Valid OAuth Redirect URI.
Hope that it helps you as it worked for me:crossed_fingers: