It's so easy to setup facebook authenitfication on your site, let's start!
1) Create new app for login on your website, choose WEB application and enter required information there:
2) Install Facebook Login Plugin, setup all the keys:
3) In Settings > Basic, fill out all the fields. You can get there your App ID and Secret key for facebook login plugin.
4) In Settings > Advanced, select API version to v2.12, set Native or Desktop app to "No" and in Security box select all to "No"
5) Go to Products > Facebook Login > Settings and set following:
Hope you got it working now.