If you pop into Facebook Messenger now you'll be able to share a song or playlist from Spotify.
Just open up a conversation, go down to the bottom and hit that ... which hides all the random apps you can use while chatting. In there you'll find Spotify and you can navigate and send something to a friend. This will require both of you to have the Spotify app of course, and if all works correctly they'll simply be able to click it to play.
The functionality is pretty simple at the moment but this is a major partnership for both brands so expect to hear more about it in the future.