I used to be a Soulseek hardcore user, mostly because I could interact with people, and easily find guys with the same music tastes, from whom I could discover lot of artists, exchange music and even collaborate on compilations.
Spotify could easily get there too, by implementing the following :
1) Develop a live chat system
2) Have some thematic rooms where you find people with the same music taste (ex: 90s hip hop, Electronica, Balkan, etc.)
Probably not too hard to implement, and this would be a much richer social feature than the current facebook sidebar.