Too much "Revenue Driver" not enough "Social"
Agree with an earlier poster that they messed up not working things out with Facebook integration.
Sharing what you're playing isn't a bad idea. Plenty of people use it on Spotify, and it is actually fairly neat, and sometimes you learn something about your friends and their tastes as well as find new music.
However, with Spotify, people are logging in with Facebook accounts. As well, with Spotify you're on a service that allows free accounts -- you don't need to pay a buck for each song you want to listen to if the name is familiar. Ping just seemed to want to drive you to buy music. I won't spend $1 repeatedly on a whim to hear music a friend is listening to, and I'm not inclined to listen to samples repeatedly.
Basically, I think Apple / iTunes+Ping would've been successful if it had been coupled with a subscription / ad-sponsored competitor to Spotify, as it was, nobody seemed to care to use it.