The service is a means of sharing music by posting tracks so your friends can see what you're listening to - as well as following your favourite artists.
Unfortunately though, no-one uses it.
And so, Apple has decided to go the way of Spotify and integrate Facebook into iTunes instead. The Facebook support in iOS6 should also make sharing music a great deal easier.
After September 30th, Apple will officially discontinue the service and no new Ping members will be accepted - nailing down the coffin on the service that was launched back in 2010.