Interesting. On Rexly, it's sort of less about "making friends," although that could happen, and more about getting social cues from friends about new music to discover. Those cues are likely to be strongest from those whom you trust, and those whom you trust are likely to be those whom you know well, unless they are liars.

When you have a friendship graph and listen info, you can both match users by their taste and recommend music based on their graph. last.fm did both and more. You can also do non-social personal and non-personal recommendations based on artist "similarities" derived from correlating user tastes. Last.fm does that too.

Another thing is that social network tracks social graph, not "friends" in the narrow sense; some of people in my social graph are likely to listen to the same kind of music than I (active friends); some are likely to listen to white noise (extended family, former close friends, collegues and people I studied with). You basically need circles!