I definitely used the inbox in the way you describe, but I’m assuming you and I are a small subset of users and most active listeners never touched the feature. So I get why they killed it.
Still, I’m baffled that no music service has been able to create a workable social integration/interface. Spotify’s QR code feature seems aimed more at in person interactions, so I don’t really get how they replace the inbox or what friction point they solve — it’s just as easy to search for a song your friend is listening to as it is to pull out your camera and scan a code on a screen. Those development dollars would have been better spent on finding a way to improve their bare bones Facebook integration.