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From wikipedia: "Spotify pays royalties based on their "market share"—the number of streams for their songs as a proportion of total songs streamed on the service."

Regardless, Spotify is free to pay whatever they think is fair and artists are free to accept or reject that compensation as they will. But if you're an artist, Spotify (and the other streaming platforms, which mostly pay even less) are not going to be something that you can ever expect to be an income source.