How could internet companys find your info if you dont give it out?

2 of my 4 local TV stations just reported about several people from our area being sued for downloading music. One of these people was using Limewire. This person is being sued by the National ociation of Recording Artists for 70,000 !! She has only used Limewire and has downloaded a little over 6,000 songs. Needless to say, she is devastated! The reporter said this is happening more and more everyday and that the Recording Artists are generally setteling out of court for 3 to 4 thousand dollars. So my question is how could they actually find your info if you dont actually sell the "pirated music"?