Among the more noteworthy numbers from Sirius XM Holdings' (SIRI -0.23%) latest quarter was its revenue from subscriptions, which grew 11% over the prior year.

With that number topping analysts' estimates, it was easy to overlook another revenue stream for the satellite radio company -- and one that's growing at a considerably faster rate than subscriptions: advertising.

That's right. The satellite radio service whose ad-free programming was once its biggest appeal to many subscribers is now raking in more than $100 million in advertising revenue annually. And while ads make up a small share of Sirius XM's overall revenue, we can expect it to keep growing.

A more "mature" satellite service sells ads

Last summer, Sirius XM's chief content officer, Scott Greenstein, said he believed the satellite radio platform had indeed "matured as a true alternative vehicle for advertisers."

"It wasn't always seen that way, and it now is," Greenstein added. "So, we're optimistic about it."

Growth picked up immediately after. And while it's tailed off from its peak of 31% in the third quarter last year, it's still coming in in the high teens to low 20s.

What allowed for the growth of advertising on Sirius was the company's push to expand beyond music -- and continue stretching out into different types of programming.

Sports, news, entertainment, talk, and live events all provide opportunities for advertising.

Faster than AM/FM, slower than Pandora

In growing its ad revenue at such a rapid rate, Sirius is bucking the overall trend in radio advertising. Advertising across the industry is mostly flat and expected to remain so in the future.

Sirius XM is not growing ad revenue as fast as leading streaming service Pandora Media (P), where ads were bringing in 23% more money than a year ago. But there are a couple of reasons why that shouldn't concern investors:

  1. Sirius XM is a more mature company than Pandora, with the satellite service now posting profits off a predictable and steadily growing revenue base.
  2. Sirius, unlike Pandora, does not rely on advertising as a primary revenue source. For Pandora, as advertising goes, so does overall revenue growth. For Sirius, the primary driver will continue to be subscriptions, which bring in 87% of its top line.

Sirius XM's advertising revenue growth is thus a nice addition and a way for the company to pad its margins just a wee bit more.

Is ad growth here to stay?

At a recent telecom conference, Sirius CFO David Frear told analysts that he sees advertising growing "faster than the national average radio market for the foreseeable future."

That's not all that bold of a statement when you consider that the national average for radio is expected to hold flat.

But Frear went on to say that the company also sees advertising growing faster than its subscription numbers. That's more telling, especially with subscription revenue now growing in the low double digits.

So what's the real potential here?

Ads now make up about 3% of the company's overall revenue, though Frear doesn't see the segment climbing above 10%.

But that still leaves the segment with a sizable runway for growth. Just maintaining its current growth rate over the past few quarters -- between 17% and 31% -- would more than double the size of advertising revenue in just five years, and potentially triple ad revenue in that short time.

In the most recent quarter, Sirius said the increase in ad revenue was being driven both by the addition of ad spots on non-music programming and by higher prices it can command on the spots available.

That's important to know. This isn't a case of Sirius simply opening up a ton of ad slots. The higher prices tell us that the demand is there.

While subscriptions will continue to be the main driver of Sirius' revenue for the foreseeable future, advertising is worth investors' attention. It could help fuel overall growth over the next several years.