Starting today, Sept. 14, Mog offers one new feature that has proved hugely popular in other on-demand music services: a free version. Rather than go with straight ad-supported or time-limited like its competitors, Mog has devised an interesting model in which users earn the free music experience.

Users get what Mog describes as a “virtual gas tank” of music called FreePlay, which starts out full and stays visible in the interface. Listening uses it up, but it can be refilled by inviting friends to join, creating and sharing playlists, and in other ways that demonstrate engagement. Mog said it intends to expand opportunities for topping up the FreePlay gauge by having advertising that rewards user interaction.

“Your social influence is a key component,” said David Hyman, CEO and founder of Mog. “Tastemakers and influencers can easily get a lifetime of free on-demand music. These models have worked extremely well for other online businesses, and it’s exciting to apply them to the music sector for the first time.”

Except for during the first 60 days, however, users will still get ads unless they decide to pay for a subscription – $4.99 for unlimited ad-free music, $9.99 to include mobile. Mog’s free service is based on HTML5 and is accessed with a browser instead of a desktop client.

“Our obsessed subscribers, and reviewers alike, tell us we’ve built the best way to listen to and discover music, and we’re so excited that we can now provide people with free access,” said Hyman.

Headquartered in Berkeley, Calif., Mog. investors include Menlo Ventures, Balderton Capital, Simon Equity Partners, Universal Music Group, Sony Music and serial entrepreneur Scott Jones.

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Photo by flickr user Marlon E, used under Creative Commons license