Web traffic up 188% compared to a year ago.

Sirius Satellite Radio may lag behind XM Satellite Radio in subscribers, but its Web site is making up for it. Since Howard Stern joined Sirius in January, traffic to Sirius’ Web site surpassed XM’s, according to Nielsen/NetRatings. In March, Sirius’ Web site traffic grew 188 percent compared to a year ago, from 666,000 unique visitors to 1.9 million in March. XM’s traffic grew 47 percent to nearly 1.7 million.

Sirius’ Web traffic first surpassed XM’s in January, when the subscription radio service had 2.3 million visitors to its site, compared to XM, which had 1.5 million visitors. Web traffic leveled off in February, to 1.59 million for Sirius and 1.58 million for XM.

Both satellite services are generating return Web traffic on par with other popular audio sites, including Rhapsody, Clear Channel Music Radio Network, and MSN Network. For the month of March, XM averaged 3.7 visits per person, while Sirius averaged 3.5 visits. Rhapsody averaged 3.8 visits per person, Clear Channel 3.7, and MSN 2.7.

At work, satellite subscribers are likely to access streaming services. Although Howard Stern’s show is not streamed online, 44 percent of Sirius’ Web site users logged in to access a media player versus 27 percent for XM.

“Satellite radio services have traditionally been thought of as linked to a specific satellite radio device. It is becoming clear that satellite radio subscribers are accessing the content they subscribe to online,” said Jon Gibs, senior director of media for Nielsen/Netratings, owned by Mediaweek parent VNU. “As these services continue to gain in popularity by adding new talent and programming, they could become a significant threat to online music subscription services such as Rhapsody and MSN Radio,” Gibs added.