I'm assuming from the phrasing of your question you are looking to get this working in dev mode.
Under your apps settings in the facebook developer center make sure you have checked the box that says 'Website with Facebook Login' and enter the url with the port you are using: http://localhost:3000
. Additionally, Make sure the app domain is set to localhost
as well.
You may also need to toggle sandbox mode to 'Disabled'. Source: a config from a working devise/omniauth/facebook app running in dev mode.