Fair use is a grey area, and I doubt that would be upheld. There are really five factors that go into fair use:
All of the cases are for making songs available for upload. None are for downloading. How the songs made it to the shared folder is irrelevant.
It is like recording off the radio.
Also, people often download songs because it's "fun," not because they want the particular song for any reason. Just the act of downloading is a fascinating thing, especially if you've never done it before.
In my case, I have fewer than 100 songs I have downloaded and kept, and almost all of them are songs I already own on vinyl or CD and just wanted quick copies to practice for my cover band.
The remainder I downloaded and ended up never listening to, or wanted to sample the work before buying. But I have yet to buy a cd after sampling. I HAVE bought concert DVD's after sampling mp3's.
If I were to go to court for "downloading," I think my motives would exhonerate me.
For crying out loud, it is just music. And music is universal. It is living art.