AllthingsD:

Sources close to the company say that Ping, which still exists today in iTunes 10.6.3, will be gone with the software’s next major release, likely scheduled for this fall. And at that point Apple’s social networking offerings will shift to Twitter and new partner Facebook entirely.

Tim Cook said as much during an interview last month:

We tried Ping, and I think the customer voted and said ‘This isn’t something that I want to put a lot of energy into.