Wow this is a bit old but the best explanation of listeners and XM radio or radio in general.
http://mfile.akamai.com/20469/wmv/xmradio.download.akamai.com/20469/leesBlog/LeeHIGH.asx
http://mfile.akamai.com/20469/wmv/xmradio.download.akamai.com/20469/leesBlog/LeeHIGH.asx