In the past few months, Spotify has seen moderate success in the United States, especially due to their partnership with Facebook. Before digital music downloads and MP3 players ran rampant, music was a primarily social experience. People gathered around a set of speakers and listened to music together. However, when MP3 players were released, music became a very private experience, with people listening to music with earbuds and headphones. Spotify makes music social again. If enabled (and Spotify pushes this), Spotify users automatically broadcast every song that they listen to on Facebook. Even if the user opts out of this, Spotify makes it incredible easy to share songs and playlists with other Spotify users.
When movies were first released, people gathered in theaters to watch them. Before that, people gathered in theaters to watch plays. Fast-forward to the late 1900s, when video rentals became prominent. People began watching movies in their homes instead of in theaters. TV shows have always been this way. However, many more people would subscribe to Netflix if it was social and people saw their friends watching Netflix movies. Now, due to some US regulation, Netflix cannot share a user's movie watching history with a social network like Facebook. But, they could at least add a Facebook share button or Tweet button on each movie or TV show episode so that users could share their interests with their friends.
Netflix (and other streaming video services) are missing on a GIANT opportunity here and should be taking advantage of this now. And Amazon should probably be looking at something like this for Kindle as well.