I've seen similar ideas before and have seen that Spotify have closed the case on the internal messaging system due to low engagement. I get why that would be the case - I already chat to my friends/family/colleagues on about 12 different platforms.
I believe that Spotify could really benefit from a Recommend function. This could work in a similar way to Facebook notifications whereby somebody could recommend an aritst/album/playlist/track to one of their Spotify connections like they can with the Share function now. This would then pop up as a notification (possibly even push notifications if opted in) saying "Ronald Dump recommends you check out 'Artist/Album/Playlist/Track Name'" with a link directly to it.
The level of social engagement is low, though if the feature was widely used, this could be followed up with a Like/Dislike feature for the receiver to use to let their friend know whether they liked it or not.
On a similar note, notifications would also be really handy when a track was added to a collabarative/shared playlist by a friend. Something like "Your Friend just added 'track name' by 'artist name' to your 'playlist name' playlist. Check it out!"
I think these would really improve the social experience of sharing music with friends on Spotify without having to leave the app.
Updated on 2019-09-09
Good ideas! It's better to post each suggestion in it's separate thread. That way, users can vote on each one of your ideas.
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