Since Facebook offloads the actual streaming to its partners, there’s no need to negotiate licenses with music industry honchos. When people see on their newsfeed that friends have been listening to a new Wilco tune, a single click on the song title actually plays the song, through the third-party service.  (It feels like it’s playing on Facebook.) If you’re not a member of the service, you automatically get signed up, via your Facebook information. This makes Facebook the prime way that these services will now sign up new users.