It really sounds to me like this is how your daughter has chosen to "stick it to you". I mean you work full-time and you are the only parent in the home. She is obviously doing this on purpose. However, as we all see she is only punishing herself in the long run.
It is obviously time for professional intervention, when making her life miserable by taking away all the usual teenage lifelines (phone, computer, social events) has not yeilded the desired result.
I say that she is "sticking it to you" because you are the one upset about her academic progress, not her. She is obviously retaliating against the chaos in her life. A good therapist will help her get to the bottom of why she feels like the best avenue to "stick it to mom" is to sacrafice her own future. She sounds like a very depressed young lady.
If her dad is still in the picture I suggest the two of you sit down with her and present a united front. Admit that you screwed up by not keeping her family together (or even creating one if you never married) but that being apathetic about her future and success is not going to change what you two did wrong. She owes it to herself to rise above your mistakes.
Get her to a counselor, one who won't buy the bull she dishes out, one that WON'T coddle you either...you both need to face the tough truth about what is really going on under the surface.
It never ceases to amaze me when we create such turmoil and virtual abandonment in children's lives and then wonder why they retaliate in self-destructing ways. This is more than needing tutors or being a social butterfly...when a child disconnects from their future success, doesn't care about what she misses in the present (social events, phone time with friends)as a punishment for that disconnect, she is headed on a downward spiral.
It is just my opinion and I know it will generate some "hate mail" but I felt it needed to be said.
I really do wish you luck and I know this can't be easy to face.