I love my friends. I trust that many of them have better taste than I do. But friendships, online or otherwise, are as much about being able to navigate the interests you don't have in common as they are sharing the things you do.
If my friends were to follow me on Spotify or share my iTunes library or, God forbid, take my Twitter #Music advice, they would be forced to confront just how much Weezer I listen to. I'm not sure a lot of my friendships could handle the weight of accepting that I love Raditude unironically.
For all that, though, most of the music I listen to now has come from a friend or an acquaintance's recommendation. What makes that different? In person, your friend is actually recommending a song to you on purpose. They know who you are, and what the song is, and they've decided that Raditude just might make your life a little better; more importantly, they know that not all their friends will be so enriched.
Trying to automate that isn't just impossible, it's antithetical to the point of recommending music in real life. Telling a friend you heard a song you know they'll love -- even when they don't end up loving it -- is basically just telling a friend you were thinking about them, him or her specifically, when they weren't around. When you give somebody a mixtape, it's never about the mixtape.
Passive sharing, whether it's Facebook's Spotify integration or Twitter's spammy #NowPlaying implementation, is the opposite of that. It's yelling to all your friends, all at once, "Hey! Think of me, because I'm right here! Listening to #FloRida perform #ClubCantHandleMe!!!"