What is algorithm behind the recommendation sites like last.fm, grooveshark, pandora?

Question

I am thinking of starting a project which is based on recommandation system. I need to improve myself at this area which looks like a hot topic on the web side. Also wondering w

Answer 1

Pandoras algorithim started with just matching specific music genres to the certain song you inputed. Then it has been slowly growing by people voting if they like the song or dislike the song, enabling it to eliminate bad songs, and push good songs to the front. It also will sneek new songs that have few votes either up or down into your song playlist so that song can get some votes.

Not sure about the other sites listed.