I've been running on site attribution surveys[1] on a handful of brands that I work with and the results have been pretty surprising. Word of mouth tends to be overrepresented and FB underrepresented though there is probably some sample bias so it's far from a sure thing.
Yeah I agree that Spotify is the best in the game right now. Discover weekly has been spot on consistently since it was released but for me 30 tracks is barely enough. I've been using turntable inspired https://www.jqbx.fm for a little while. The social part of it gives it the extra push of randomness I need to discover music I wouldn't have stumbled on otherwise.
[1] https://www.zigpoll.com/examples/attribution-survey