The following is an exerpt from an older topic in the Spotify Community:
If you created a Spotify username and password when you signed up, log in using those and disconnect by going to Preferences and clicking 'Disconnect from Facebook'.
You can access preferences on OS X by pressing Command(⌘) and comma(,).
You can access preferences on Windows by pressing Ctrl and P.
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Hmm, maybe you can add a username here.