A lil' help on MSN - SN??

I started MSN about 2 weeks ago, so I'm quite new to MSN. All my friends say "sn", but what is this "sn"??? when I played a song called "After Love", one of my friends came up saying "AFTER LOVE!!!! I LOVE THAT SONG~~~ its my sn!!!", and another friend says "U dont rly spend much time on sn, do u?" I REALLY DONT GET WHAT THIS SN MEANS, SO COULD YOU PLEASE HELP!