If you opted for Play App Signing you need to generate your key hash based on the certificates generated by Google Play.
If you end up here after endless debugging, not knowing why your Facebook login still doesn't work after you put all the key hashes you could find in that Facebook app dashboard... I feel you. :(
Been there done that.
Two painful days trying to debug and see why the keys I'm getting from the keytool export are not a match for the app version in the store.
Either a total brainslip or just got over this without acknowledging it but at some point you can opt-in for Play App Signing which takes care of signing your app. If you did this the key needed to be put in the facebook app dashboard needs to be taken from Google Play Console, not from your local key store.
Go to your app page in the console, search for App Signing and there you'll find the certificates you'll need to convert to base64 and put in the Facebook app dashboard.
That's it.
Thank you for reading 😄