Hugs and understanding from me mama
When our kids turn into teenagers lots of changes happen. I found during this time communication being a key. Our kids enter a completely new stage of their development and it is playing havoc with the all the aspects of their lives.
Just like with toddlers it becomes important to carefully pick your battles.
As in MySpace case - may I ask why is it important to you that she does not have an account? Could the compromize be reached, where she would actually feel like she can ask your opinion on what kind of information she *should* reveal and what she should not? MySpace in not evil in and of itself. So can you use this incident as a teaching moment AND listen to what she thinks about it? Let her talk about it and listen (even if you do not agree AT ALL with what she is saying). Hear her out - not only during the argument when the emotions are hightened, but may be during your lunch together.
Obviousy it is important to her, so instead of just forbidding it - find out WHY it is important to her. If you honestly and attentively hear her out - she may just repeat the gesture - hear YOU out.
Grades dropping - I would not be a good advisor in this respect. I am not at all in agreement with current school system and am not at all surprised that majority of kids nowadays loose interest in school...