edit: one good reason I see to start off with music (besides the fact that it makes sense for iTunes) is that it's more common in the music industry to "follow" an artist, than it is for TV or movies. So, right from the start anybody will have people to follow while they're building their friends network.
I bet 'gifting' people in your Ping friends network with music (and apps, books later on) will be a profitable feature for Ping, when Apple implements it.
When gifting right now, I don't think you have any way to know if the recipient has or not the song/app. I haven't gifted songs too much, so I'm not sure what happens in that case. Ping might be able to avoid this situation.
A ready-made social network, all the network infrastructure in place, users with credit cards at the ready, owned by a highly successful tech multinational....the recommendation on APPL is 'buy'!
Ping seems to find that requirement exactly. Watch this get used on the TV side pretty quickly if it takes off.
This vs. on facebook where people are wary of TMI and people who spam their feed, this is all about doing that.