To check this: According to Spotify, their 24 million users listened to 4.5 billion hours of music in 2013. This gives us 166.667 hours per user per year. If you take an average song length of 3.5 minutes, this results in 2857 plays per year, which, if you assume Spotify's number of $0.006 to $0.0084 (let's take 0.0072 USD) per stream, this results in 20.57 USD per user per year. Wow, very close, it indeed looks like a user generates around $20 per year.
If the entire US population would use Spotify, this results in roughly $6.5 billion per year.
Of course, that's a bit rubbish, let's use the amount of households in the US. Around 120 million. So that's more like $2.5 billion.
If we assume an average household income of $45'000, this would support around 55'000 individual household incomes in the music industry (artists, studios, engineers, right holders and so on).
Of course, since most money flows to the mega stars. It would support much fewer people. So let's take 31'400 people. Why? Because it goes well with the US population figure of 314 million. It's exactly 0.0001% of the US population.
Still thinking about making a living with music?