I think Ping would have been more successful out of the gate with Facebook integration but their partnership with Twitter will eventually prove to be a better long term strategy.
With Facebook, Apple would have been stepping into Facebook's music business which would have had them competing with their partner. Twitter is a more lean social network where Apple's Ping service can enhance and complement Twitter and vice versa.
Given that Apple hasn't dropped Ping yet and has been very quiet about it, I think that they're preparing for a relaunch or a major rethinking of the concept. Having bands and music communities built directly into iTunes is an excellent idea whose time will come if implemented correctly.
I believe that Apple should work directly with Twitter by building it into iTunes. Instead of a Ping account, you'll be able to sign in with your Twitter login and follow your favourite artists who are already very active on Twitter.
Twitter in turn will cede more in depth artist profile pages to Apple. Artists will be able to update upcoming concerts and music releases that iTunes users can subscribe to, to be notified. Users will also be able to share their music purchases, listening habits with people who choose to follow them in order to discover new music.
Ping is a great enhancement to iTunes but it's been implemented in a way that necessitates users and artists to manage and update yet another social network. This is why it has not become popular: it's trying to compete with ingrained existing social networks. By allowing Twitter to be built into iTunes and then offering services better offered directly in iTunes, Ping can become very useful and convenient.
