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Maybe but they'll get promotion within facebook. Lots of users and they can always split if FB goes under.

Why would it mean that less people will sign up to Spotify? I'd think that being pushed in everyone's social feed and making a signup be one click away and no text fields to fill out, this will mean they'll gain a massive amount of new users.

The certainly did this not just for fun. Would be interesting to know the "incentives" involved...