Spotify can no longer display your Facebook username becuse you deleted your Facebook account. If you used Facebook to signup to Spotify, it would mean that you didn't specify a Spotify username and have been assigned a username of random numbers and letters.
You can change your username by creating a new account with the username of your choice. Once you've done that, you can get in touch with the Spotify support team using the online contact form, and they can transfer everything over to your new username/account.
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