Facebook is requiring app review to allow new apps to query public Facebook content using the API. This means that you have to submit your app for review (for the ‘Page Public Content Access’ permission) and get accepted, for the plugin to import content from Facebook, using your App key and secret.
However, the plugin will provide you a functional (preapproved) app id and secret that can be used to import public Facebook content. More info on plugin config + Facebook app approval, here.
