After I expressed my disappointment with Ping yesterday, a helpful reader pointed me to Like.fm as an example of the promise of social music.
Here's how it works. First, download and install the free plug-in for iTunes. (There's also a WinAmp version for you holdouts.) Whenever you play a song from iTunes, Like.fm posts the title and artist on a public Web page. Other users can then click on the "play" button on that page and if a video of the song exists on YouTube, it'll start playing. If you link Like.fm to your Facebook profile, you can create a new "Music" tab on your profile that lists all the songs you've played. Here, your friends can click on any song title on that profile page and their browser will open to the Like.fm page, where the YouTube video will begin playing in a small window.
There's also a separate plug-in for the Chrome and Safari browsers. Install it, and any time you play a song on YouTube or Pandora (other sites are coming soon), it will post that song to your Like.fm profile.
The service isn't perfect--it didn't recognize several songs when I played them on YouTube, and currently the only way to find friends on the service is if you both link your accounts to Facebook. But conceptually, this is how social music is supposed to work: you can actually listen to music your friends are playing, rather than being guided to 30-second samples in an effort to get you to buy the download.