Why is it "ewww" for you? What would be a better solution? What are the pros and cons of this decision for Spotify, for FB and for the user?
Pro: some users like the integration and the social features that it gives them. This doesn't explain why it's now mandatory, or why it's become mandatory right now.
Pro: Has Spotify done some kind of deal with Facebook, and Facebook insisted on this? I put "pro" since I assume that Spotify also got something out of the hypothetical deal.
Con: There are some users (like myself) who think that facebook knows enough about me already, and really do not want to integrate them into anything. I like the face that the web is not an extension of the facebook machine.
As an existing Spotify user this policy does not apply to me (yet?) but it's repellent. As a paying Spotify user I don't feel that I would be obliged to put up with that. I don't complain about FB too much since hey, I didn't pay anything for it and no-body actually forced me to use it. I know, "if you're not the customer, you're the product" - but with Spotify, I bloody well am the paying customer, they've got my money, they don't need to sell me to Facebook. They can take their Facebook integration and GTFO.
Edit: a friend has suggested that Spotify are angling to be bought by Facebook, and this will help them integrate for that.
It's worth noting that Spotify used to have optional facebook integration, i.e. "the standard Facebook - or - Default sign up page", and they have taken the effort to remove this for new signups. There must be some reason for actively removing this choice, and I don't think that "mass-market users are confused by having 2 different choices" is it.
If a Facebook account is mandatory (at the moment it isn't), they might expect a greater number of users to link their accounts. It doesn't seem unreasonable to expect that the number of people who additionally link their accounts will exceed the number of subscriptions lost from people who refuse to have a Facebook accounts. If a greater number of people linking accounts => increased Premium registrations, then this could very easily be a purely business decision.
Also, this is a statistical relationship - there will be users who were likely to buy premium membership but don't like facebook, who are now less likely to buy. Or who are now reconsidering their existing spotify subscriptions.
Facebook have an increasingly bad reputation. Spotify, now it has made Facebook the only way to sign up, has tarred its own reputation by becoming so closely tied to Facebook.
Beyond this they have locked out users who do not have a Facebook or do not wish to tie an essentially private Facebook account to a public service.
Looking beyond the sign up page, they are also increasingly pushing Facebook and other social media on to Spotify users with no way to hide some of these features. This, for me at least, is just adding distracting fluff to the Spotify UI.
Considering how they got 2 million paid subscribers without this functionality it is baffling that they would essentially disable completely a sign up mechanism that was working perfectly well.
As a premium Spotify user I will be switching to Deezer next month if a u-turn is not made on this decision.