The Facebook Platform uses OAuth 2.0 for authentication and authorization. You need to setup Facebook Application and connect this application to your blog and authorize it.
Create a Facebook App for your website.
Login to your Facebook account.
Go to the Developers page: https://developers.facebook.com/
Click “My Apps -> Add a New App” menu item.
Fill “App Name” and “Contact email”
No need to add any products yet, just click “Settings”->Basic on the left.
Select “Website” in the list.
You should see the “App Dashboard”->Add a Product page. Please click “Set up” in the “Facebook Login” product box.
Choose “Web” from the “Platform” selection
You should see your site’s URL. If not, please enter the URL of your site to the “Site URL” field. Click “Save”.
Click “Settings” under “Facebook Login” in the left side menu. Please enter the URL of your SNAP Plugin “Accounts” page to the “Valid OAuth redirect URIs” field. Click “Save changes”.
Please go to your WP Admin panel and click on “Accounts” under the SNAP|Autoposter in the left side menu. The URL of that page is the “URL of your SNAP Plugin “Accounts” page
Important] If your site uses httpS (SSL) protocol you can skip this step.
If you are still using http, please turn off (Set to “No”) “Enforce HTTPS for Web OAuth Login”
All Facebook apps are in “Development” mode by default. All posts will be visible only to to Admins, Developers and Testers. You need to take an app “Live” if/when you would like your posts to be seen by everybody.
Click “App Review” on the left. Change “Make APPNAME public?” switch from “No” to “Yes”.
Click “Save Changes“.
Your App is ready to use.
Connect Facebook to your WordPress.
Login to your WordPress Admin Panel, Go to the Settings->Social Networks AutoPoster Options.
Click green “Add new account” button, select “Facebook” from the list.