Some setup you must do manually if you haven't yet:

1. Ensure you have overridden or uncommented the routes for generated omniauth callback controllers in your routes.rb.
   For example:

   Rails.application.routes.draw do
     devise_for :users, controllers: {
       omniauth_callbacks: "users/omniauth_callbacks"
     }
   end

2. Update the devise facebook omniauth APP_ID and APP_SECRET in "config/initializers/devise.rb"
   after registering the Rails application on: https://developers.facebook.com/apps/
   For example:

   config.omniauth :facebook, "APP_ID", "APP_SECRET"

3. Your omniauth callback URL will be: `<host>/users/auth/facebook/callback`. Please update
   callback URL in Facebook after registering your omniauth application.