My suggestion would be for you to create a new account with a different email address from the one you use for facebook (past or present). That will give you a Spotify username of your choice, so you won't have to rely on facebook anymore.
Once you have this new account, contact customer support and they'll help you migrate your Premium privileges (if you have them), library, playlists, friends, etc. to this new account.
With that done, you'll be able to deactivate your facebook without worries.
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