One of the main differentiators of Google Music is its sharing functionality: if a friend of yours shares a song on Google+, you get a free listen of the entire track. Now those tracks are being collected in a single place with the addition of an automatic playlist called "Shared with me." Google likens this feature to an email inbox, and that doesn't seem too far off the mark — there's a counter of songs that you haven't yet listened to, and those new tunes appear bolded in the playlist. Of course, there's also a link to buy the song directly if the single listen doesn't whet your appetite. The feature, like Google Music itself, is currently only available in the US.