I'm using 3G Watchdog to track my 3g traffic and noticed that it does not monitor my Grooveshark app. Do you know why? Got a Sniffer that works better?
Grooveshark might not be using that much data; it keeps a cache of songs you have listened to recently so it doesn't have to re-download them each time. So if you're listening to the same songs all the time, they will probably only be downloaded once, same with offlined songs. To see if the app really sees what data Grooveshark is pulling, go to
Grooveshark->Settings->Clear Song Cache. Then go listen to a playlist or two and see what happens :)
P.S. You can also edit the size of the cache on that menu!
If all else fails, i use Phone Usage, and it tracks Grooveshark pretty successfully: