To add your own facebook login system to the script you need to create an facebook app. for that follow below steps
login to your facebook and go to https://developers.facebook.com.
now click on my apps button
now click on create app
select app type, select consumer and click continue
give info for your app
now go to facebook login settings area
now you have to setup your facebook callback url, your facebook login call will yourdmain.com/facebook/callback, do not forget to change your actual domain with yourdomain.com
now go to settings > basic settings
add your app domain and your email, remember your app domain must be your website url.
add platform select website
add website url to Platform
now you have to add your facebook app id and app secret to your website admin panel, for this go to “General Settings > third party scripts”
if facebook login is not enable, then enable it and add facebook app id and app secret
that’s it, facebook login setup is done.