A key metric not mentioned in the comments is premium average revenue per user (ARPU). It's consistently decreasing over the last 3 years:
2015: € 6.84
2016: € 6.20 (9% decline)
2017: € 5.32 (14% decline)
Also, the number of hours consumed per MAU as of December of each year is also increasing. I should have taken the average over the year for both but I am interested in the trend.
2015: 191 hours per year (17.4 billion/91 million)
2016: 217 hours per year (26.7 billion/123 million)
2017: 253 hours per year (40.3 billion/159 million)