Thanks John....well as you know this is a difficult age to begin with and then add a few stressors and it is hard on everyone. Then, there is the balance you need to obtain between giving her some independence but still being a safety net if she falls.
Here's what I would recommend....seek help with your "safety net" role. First, since she has always struggled with grades, ask for an IEP (special education) in the category of Other Health Impaired (OHI) for her ADD diagnosis. She will be provided with a resource teacher who will take on the responsibility of her school performance. You should be taken out of the equation on that one because it causes family strife and it should be the schools role anyway (since grade have always been a struggle). You do not need to be the disciplinarian regarding school.
Then, it sounds like she is going through some adjustment changes (questioning her identity and problems with friendships)....this is typical for his age but, again, if you suspect that these adjustments are interfering with her function at school, she may benefit from an outside person to talk to....a therapist. Here is a list in your area. http://therapists.psychologytoday.com/rms/prof_results.php?sid=1356675095.1353_14483&city=Rancho+Santa+Margarita&county=Orange&state=CA&spec=5
You can also use the therapist to help you have a dialog with daughter about the rules (about time on FB and IPhone) and support you in your enforcing of the rules.
The most important item to avoid loosing your daughter is to keep communication open. So taking yourself out of the conflictual areas (grades and adjustments) and having healthy discussion and activities together (with help of therapist if necessary) should be your goal.
Be rest assured, the difficult adolescent years (14 and 15) do pass and adjustment does happen towards happier times ahead.
I hope this helps....warm regards