Im fairly sure they manage because they have so many subscriptions from people that barely use it.
They basically pay out per song played - and server costs are also largely dependent on active users. So they balance out a very active person that might incur 15$ in cost with 5 inactive people that incur not even a dollar.
Then there are people like me who do use Spotify a lot, but I'm mostly listening to the same stuff most of the time. Unless I'm trying to find new things or listen to a podcast, it's most likely all cached on my device.
I guess I don't really know exactly how their model works, but I'm guessing that is only for independent artists. I bet they paid big bucks to the major record labels.