It's easy as 1, 2, 3.
1. Open ShareMusic
2. Add a personal message (optional)
3. Press the Share button
It also lets you control your music playback right within the app and change the message format.
It will share these information on your wall:
You need to be logged in a Facebook account. Artwork and link to the iTunes Store might not represent the current playing song if the album/artist information is not found.
And iTunes must be running.