Try this ---
https://www.facebook.com/help/community/question/?id=589302607826562
1 - Click Apps and then select your app.
2 - Click the Settings button on the left side of the screen.
3 - In the Basic settings, click the Add Platform button below the settings configuration.
4 - Select Website in the platform dialog.
5 - Enter your URL (localhost works here).
6 - In the App Domains text input, add your domain that matches the one in the URL.
7 - Save your settings.