Use OmniAuth middleware with Facebook inside of an engine. Example: https://github.com/pablomarti/simple_user/blob/master/lib/simple_user/engine.rb

engine.rb

module SimpleUser

  require 'rubygems'
  require 'omniauth'
  require 'omniauth-facebook'

  class Engine < ::Rails::Engine
    isolate_namespace SimpleUser

    middleware.use OmniAuth::Builder do
      provider :facebook, ENV['FACEBOOK_KEY'], ENV['FACEBOOK_SECRET'], :scope => ENV['FACEBOOK_SCOPE']
    end

  end
end