Satellite radio isn't just a big hit with audio buffs on the go. Both XM Satellite Radio (NASDAQ:XMSR) and Sirius Satellite Radio (NASDAQ:SIRI) are becoming hot spots online, too.

Yesterday, Nielsen/NetRatings reported traffic statistics for Sirius.com and XMRadio.com. The numbers are as encouraging as they are tantalizing in terms of potential new revenue streams.

Even though XM has more subscribers than rival Sirius -- 6.5 million versus 4 million -- Sirius has been the more magnetic online attraction since Howard Stern came on board in January.

Last month, Sirius' online site drew 1.9 million unique visitors. That's a 288% surge from the 660,000 sets of eyeballs that Sirius.com drew in March of 2005. Even though it's a dip from the 2.3 million cybersurfers who flocked to the site in January when Stern's show first aired, it's a significant sequential improvement from the 1.6 million visitors that stopped by in February.

XM drew 1.7 million visitors to its website in March. That's fewer than Sirius, but it still represents a respectable 47% gain in traffic over the past year.

The satellite radio sites have been sticky, with the average visitor hitting a site nearly four times a month. By spending an average of 13 to 15 minutes a month on the site, the traffic is nearly comparable to what music subscription sites like RealNetworks' (NASDAQ:RNWK) Rhapsody command.

Sirius and XM offer online streaming to radio subscribers. Still, one would think that most of the visitors are either curious potential customers hitting up the site to learn more about satellite radio, or active members seeking programming content updates.

With all this traffic, it's really only a matter of time before XM and Sirius monetize their websites as a way to achieve positive free cash flow sooner rather than later. It wouldn't take much for either company to side with either Google (NASDAQ:GOOG) or Yahoo! (NASDAQ:YHOO) to run targeted text ads on their briskly viewed pages in exchange for a lion's share of the ad revenue. It may also inspire corporate sponsors to approach Sirius and XM for more lucrative stand-alone marketing campaigns.

In other words, this is yet another factor to consider for those who feel that satellite radio will only be about the monthly subscription fees. With inroads being made to increase everything from sponsorships to e-commerce to content syndication to interactive marketing, there are plenty of reasons why satellite radio is still looking up at the stars.

XM was singled out as a recommended stock to Rule Breakers subscribers last year.

Longtime Fool contributor Rick Munarriz is a Sirius subscriber, but he does not own shares in any of the companies mentioned in this story. The Fool has a disclosure policy. Rick is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early.