Back in March, music recommendation service Pandora promised to release monthly audience metrics, as it looks to woo potential advertisers with its figures.

True to its word, it has been doing just that. It declared 49m active monthly listeners for February, representing 975m listening hours. This rose to 51m and 1bn respectively for March, and its latest figures for May show a continued upwards trajectory, with 53.3m active listeners for the month (up 52% on last year), amounting to 1.10bn listening hours, up 87% from the corresponding period in 2011.

Its total share of US radio listening hours in May was 5.80%, up from 3.08% at the same time last year. Pandora calculates its radio market share using a seven-day trailing period from the end of the month, which for May included the Memorial Day holiday.

In other Pandora-related news, the company has announced that Mazda has joined the growing roster of car manufacturers that are planning to integrate Pandora into their radio systems. The first models are expected to be available at dealers beginning in the fall of 2012. The Mazda in-dash entertainment system will support Pandora functions such as thumbs-up, thumbs-down and track skip.

To date, Pandora has announced partnerships with 19 car-makers and seven aftermarket manufacturers to offer personalized radio via the in-dash entertainment system. There are more than 50 models that feature Pandora currently available at dealerships, and the in-car connectivity is made possible via a paired iPhone running the Pandora mobile app.