A big part of the algos was the "Music Genome Project"[1] as well. This allowed their recommendations to be about the music itself instead of just what other people also listened to, leading to some amazing music recommendations/discoveries.
At least someone's still at it, and all this helps me find my notes from that time.
https://github.com/LDugar/MusicGenome/raw/master/Decoding-th...
[0]http://www.pandora.com/about/mgp [1]https://github.com/omgDB