Friends (the real social network)
Radio, motion pictures, television, or other forms of mass media
Commercial Streaming Services (Spotify, Pandora, etc)
Music literature and/or periodicals (online or in print)
Attending local performances / music events
Vinyl Community Social Networks
Crate digging for intriguing mystery albums
Recommendation engines like Gnoosic, Allmusic, etc.
Torrent search aggregators/featured private torrents/manual searches
Recommendations from your local indie record store gurus
Following independent music blogs or sites like Hypemachine and Pitchfork
Subreddits like r/listentothis
Web forums, non-music specific