Ben,
Brian's theory makes more sense than the spam theory. I went back and checked on the artists in question and none of them had more than four fans, none of them asked to be my friend and none of them had more than six 'favorites'. Basically, they just added my song to their favorites list.

On myspace, I don't accept just any friend request and as soon as I see any spam, I will block them and delete them from my list. Haven't got any porn requests as of yet, how did you get so lucky?