Music is often best enjoyed when shared, and Spotify nails social better than any other program out there. Instead of stubbornly attempting to create its own social network a la iTunes and "Ping," Spotify connects directly with Facebook and Twitter and scrobbles to Last.fm.
You can see all your Facebook friends who have Spotify in a sidebar within the app, and easily listen to playlists that they've created or see what their top albums and artists are. You can even make collaborative playlists that everyone contributes to.
If you're listening to a great song and want to tell the world about it, you can send a Tweet directly from Spotify with just a couple clicks. And if you have a particular friend in mind who needs to hear a track, Spotify will let you send it to them via its "Inbox" feature. The next time they open the program, they'll get the song along with your message.