There are some other good answers here. However I want to add information about how to create a Test app (as recommended by Facebook).
Go to the page to manage Facebook apps (you, the admin must be logged in): https://developers.facebook.com/apps/
Hover over the block containing your live Facebook app and click on the three dots button that appears on the bottom right corner. Then select Create Test App from the menu.
The test app will be created with a snapshot of your production app’s settings. After this, you can independently edit the settings of your test app without affecting your production app.
See the documentation here: https://developers.facebook.com/docs/apps/test-apps/
Regarding the rest of the setup:
- keep your app in Development mode
- go to Settings -> Basic (left hand menu) and
- enter “localhost” for the App Domain
- change your Site URL to the localhost version (e.g. http://localhost:3000/users/auth/facebook)
Here are some (Jan 2022) screenshots of my test app settings, which I just use for Facebook login.