Citing foreign media, Eurogamer reports that Nintendo no longer allows users to log in and create accounts via Twitter or Facebook. Starting October 25 of this year, users will only be able to log in to their Nintendo accounts through their tied Google or Apple accounts.
After the new policy takes effect, if you wish to sign in again, then simply enter the email or user name that is bundled with your Nintendo account. Naturally, users can also use their Google and Apple accounts to sign in.