Cloud Drive Sidesteps Legal Obstacle to Ubiquitous MP3s

Amazon will monitor and shut down such usage as against their T&Cs, not because it's "piracy" per se, but because it would cost them more money to deliver the audio than they'd be making from mp3 and storage sales.

You can of course already do exactly what you propose with various other services, though again many would shut you down if half the internet tried to access them for equally pragmatic reasons. Small darknets based on social networks are wholly possible though via nearly anything that can transfer files e.g. gmail, google docs, dropbox, IM, samba, apache, swapping USB drives, etc.