Michelle and Peg have given you good advice. I'll add that in todays social climate it's not just teens who mouth off. Yes, she should say she's sorry. How do you know her apology was meek or insincere?
Thinking about boundaries and putting them in writing for both you and your neice might help. Give a copy of your boundaries to her parents. Included in this list is that you will talk with her parents anytime you are concerned.
I understand why she got mad. She didn't know you'd tell and she felt hurt. If I were you I'd tell her that you're sorry that you hadn't talked with her before you talked with her parents.
Apparently the deal you made with your neice needed to be discussed more fully, if an F negates getting the PSP. It sounds like the only requirement that was stated is that she have a C+ average. She could have that even tho she's flunking one class. By the way, flunking one class is a learning experience and does not need to be punished. Discussed without anger in an effort to understand, yes. Does she need some extra help? Is she just goofing off? That sort of thing. Consequences related to it, yes. But not punishment.
I don't think I would've told her parents about the F grade for several reasons. They will know when she gets her report card anyway. Why do you need to tell them in advance? It causes her to lose trust in you. And it sounds to me that this is the way you are expressing disappointment and perhaps anger at her. The issue is between you and your niece. If I were her I'd expect you to talk directly with me, especially since you are in her MySpace as a friend. Friends don't tell.
This is why you need to clarify your role in her life. As to MySpace the two of you can clarify boundries by talking about what each of you expects if you have access to her space. If the two of you can not agree on a boundary then she should not allow you access to her MySpace.
As to her outrageous comments. Teens are very emotional. So are other ages. You'll be getting it from your daughter (if you haven't already been told that she hates you) once she is in school because she will hear those words from her peers. Beating her down will not cause her to stop. Unfortunately expressing anger in that manner is the way of our society. Beating her down will increase her anger and most likely escalate the behavior. Yes, she should have a consequence that will help her learn to not use outrageous language.
I think that your neice has calmed down by now but is still anxious about your reaction to her anger. Anger is a normal defensive reaction and so she may still be feeling anger to protect herself from your anger.
In this situation you have not talked with her. which is a consequence. Her father had her tell you she's sorry. Discipline is over.
You can be sure she didn't mean that you are nothing to her. It is accurate that you are a tattle tale since you were acting as a friend and had not told her you would be telling her parents. She is hurt and you both share responsibility for what happened and each of you is responsible for your own feelings. You can decide to remain angry or you can attempt to understand your niece, a part of which is to talk with her. Be sure to let her know what you will do with information learned on her site.
Keep in mind that it would be helpful to your relationship and thus your ability to help her know and understand about life if she can trust you not to go to her parents. In reality you only need to tell her parents if she may be harmed by what she's telling you. And it's best to tell her that you will talk with them when you feel it's necessary. Be honest and upfront in your relationship.
By the way, what is a PSP?
I hope that you will think about what has been said and not take offense. Some of what I've written about I only know because I've experiences similar situations. I had a teen age daughter and worked with teens during my career.