Reasons:

1. It's free.
2. I can pause and resume songs.
3. I can listen to whatever I want, instead of settling for songs that sound similar.
4. I can create my own playlists and order songs the way I want.

cankoklu@grooveshark

I will still keep scrobbling to my last.fm account but I've been listening to locally stored music less and less these days.

Btw, Grooveshark does have some ways to go in terms of integrating itself to social networks.. Right now you have to share something manually and on a per network basis.. They will probably fix that soon (I hope).