Not A Meaningful Number.
"To date, Spotify says it has paid $3bn in revenue to rights' holders."
This is not a particularly meaningful number. It might indicate Spotify's expense for the basic product they sell but as an indication how much real, individual, specific rights holders earn, it is more deceptive than informative.
In Re The $3bn Number:
1) How many streams has it paid for?;
2) How many artists have gotten a piece of it?;
3) How many months did it take to earn (or disburse, depending on your point of view) such a sum?; and so on.
Other meaningful numbers would be:
1) The earnings per artist, per track, and per listen, ranked by percentile;
2) The income per user per stream, track, and artist, ranked by percentile;
... and so forth.
Note that the above categories can be further subdivided by source of revenue i.e. by paying and by non-paying, free-tier users.
These would be meaningful numbers.