Pandora has gained 5 million users in the past 3 years, while Spotify has gained 100 million

Pandora is down a CEO, and it's not hard to understand why the company's board and investors might have been itching for a change.

To put it simply: Pandora hasn't been doing great, particularly compared with the competition. Though the company helped pioneer and popularize streaming music, it's watched as its ad-supported, radio-style model has been overtaken by on-demand subscription services like Spotify and Apple Music.

This chart from Statista helps put the shift in perspective. Back in December of 2013, Pandora said it had 76.2 million active users each month. At last check, in December 2016, it had 81 million, most of whom use its free tier. Over that same time period, Spotify shot up from about 40 million monthly active users to about 140 million. And its number of paying subscribers surpasses that of Pandora.

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That comparison isn't exactly apples to apples. Pandora is only available in three countries, while Spotify is available in 60.

And it's not like Pandora has been sitting still. The company recently launched a pair of on-demand offerings. And 81 million active users is nothing to sneeze at.

But there's a reason the company has been surrounded by M&A chatter for months. A recent investment from Sirius XM might help, but the company's search for people willing to pay continues.

Samantha Lee/Business Insider/Statista