The social aspects are important for a product like Nightingale. This page tries to show the possible channels of social interactions, the user could do with music in Nightingale.
Services: Last.fm, Facebook, Libre.fm
Purpose: uhm, no special… Just sharing with friends (and none will look at it)
Services: Facebook, Last.fm
Purpose: highlight a track, to show it to your friends in your long scrobbling list or as reminder for yourself, that you like the track.
Services: Twitter, Facebook
Purpose: really recommend a track to your friends, maybe write a comment to it. This is more fun with a link where you can listen the track. We need to divide between two cases:
Services: WordPress, Tumblr, Blogspot
Purpose: ability to write down an album review, or artist evolution for example.