discourse/lib/auth/facebook_authenticator.rb

class Auth::FacebookAuthenticator < Auth::ManagedAuthenticator

  AVATAR_SIZE ||= 480

  def name
    "facebook"
  end

  def enabled?
    SiteSetting.enable_facebook_logins
  end

  def register_middleware(omniauth)
    omniauth.provider :facebook,
           setup: lambda { |env|
             strategy = env["omniauth.strategy"]
              strategy.options[:client_id] = SiteSetting.facebook_app_id
              strategy.options[:client_secret] = SiteSetting.facebook_app_secret
              strategy.options[:info_fields] = 'name,first_name,last_name,email'
              strategy.options[:image_size] = { width: AVATAR_SIZE, height: AVATAR_SIZE }
              strategy.options[:secure_image_url] = true
           },
           scope: "email"
  end

end