Log into another Niantic game, like Pokemon GO, with your Facebook account and link a Google accounts in its settings. You can then use that Google account to log into Ingress. See also:
Facebook has reactivated Ingress this morning so you should now be able to use Sign in with Facebook to sign into Ingress; however, after signing in, if you haven't already it may still be helpful to link an alternative login method (like Sign in with Google or Sign in with Apple) so that you have multiple login methods to choose from going forward.
Comments
Log into another Niantic game, like Pokemon GO, with your Facebook account and link a Google accounts in its settings. You can then use that Google account to log into Ingress. See also:
Facebook has reactivated Ingress this morning so you should now be able to use Sign in with Facebook to sign into Ingress; however, after signing in, if you haven't already it may still be helpful to link an alternative login method (like Sign in with Google or Sign in with Apple) so that you have multiple login methods to choose from going forward.