Spotify has announced on it's Community blog that it is "deprecating" the Inbox/Messages feature in the Spotify app.
"Extensive data analysis has shown that this feature has very low engagement. The huge disparity between the use of the feature and the manpower required to maintain it doesn’t merit keeping it running, so we've taken the difficult decision to remove it."
Instead, Spotify hopes that you'll use third party messaging apps to share music, and promised more outside integrations and new in app features. "We’re always trying out innovative ways to use the app, so stay tuned for new sharing features in the future."