An official statement from Spotify says:
To us, this integration is all about creating an amazing new world of music discovery. As most of our users are already social and have already connected to Facebook, it seemed logical to integrate Spotify and Facebook logins. We already use Facebook as part of our backend to power our social features and by adopting Facebook?s login, we've created a simple and seamless social experience.
The problem is, that doesn't actually address the concerns of people who neither have nor want a Facebook account. Adding an option to sign up using a Facebook login makes signing up easier.
Requiring someone who doesn't have a Facebook account to create one and then manage privacy settings, including keeping up with any changes Facebook makes, puts up a barrier. Dismissing their concerns about Facebook is the opposite of addressing the issue.
To his credit, a second tweet from Ek conveyed a more sympathetic tone, saying, "We'll try lots of things, and probably screw up from time to time, but we value feedback and will make changes based on it."