Create a new app at: https://developers.facebook.com (for more details visit: https://developers.facebook.com/docs/apps/register )
Go to : https://developers.facebook.com/apps/
Select your app
Go to settings/basic and add the site url to Website - Site URL
Go to products
Add Facebook login
Go to Products/Facebook Login/Settings
Set Valid OAuth Redirect URIs to : {your site url}/DesktopModules/DnnSharp/Common/WebHandlers/DnnSharpFacebook.ashx/
Where to find Facebook App ID and App Secret
Go to : https://developers.facebook.com/apps/
Select your app
Limitations:
Facebook post permissions
All Administrators, Developers and Testers can automatically post, for all other users you must go at https://developers.facebook.com , go to Status & Review - Add items to this Submission and add publish_actions. Facebook must review and approve this featurs.
Requires HTTPS
Side notes
The Facebook Login action also creates the FacebookId profile property. And stores the created users Facebook client id in it.