Was anyone a Sabrina the Teenage Witch fan?

I have been watching the reruns and really enjoy it. When it was on I didn't realize she had a friend named Jenny in season 1. In season 2 Jenny was not there, but replaced by a new girl named Valerie. Does anyone know why she was written out or left the show? And then in season 4 Libby and Valerie were both gone. I think she said Valerie moved to Alaska, but I forgot what happened to Libby. Anybody know? Thanks:)