I have iTunes open 99% of the time I'm using my computer, playing music while I work. With the iTunes sidebar I can comment on songs, like songs, etc.. while I'm using iTunes. If I'm using iTunes and I hear/buy a song and want to share it, that's the appropriate place to do so. I'm already in the desktop app, so why should I have to goto the web to do what's right in front of me?
iTunes should be the publisher, sending the information I enter to FB/Twitter. That exposes it to the world, where my FB friends can comment (or even better, play the song preview on FB, buy, like, etc..). All that information (again, if Ping/FB integrate with each other) would then be dumped back to Ping so if I'm on iTunes I can see what my friends said or bought, or if I'm on FB I can see the same information.
Basically the information exchange should be two-way, so if I'm in iTunes on Ping or on FB I see the same songs, comments, likes, etc... If I'm on FB and want to buy a song, iTunes is going to have to open anyway, so at that point the discussion is moot.
You're right though about Ping/iTunes being slow... perhaps iTunes Preview will expand to allow Ping integration (if it doesn't already) and give you a quick web interface for when iTunes isn't open... but if you're buying or listening to music aren't you going to have iTunes open already?