i see a few problems with your situation. first of all dont get offended by a lot of the things everone else on this post have been saying. for the most part they all have very valid points.
#1 you should have asked her out right away. she may have been attracted to you at some point, she wanted you to make a move but you didnt. and because you waited so long to make your move she may have put you in the friend zone.
#2 i dont believe she even had a bf. girls use that excuse all the time, and most of the time its just a s.h.i.t test just to see how you'll react. you failed miserably, now she thinks you are a creepy, needy guy, who might turn out to be a stalker. so there is probably no chance you will ever get with this girl. you should have acted like it didnt matter.
#3 the facebook deletion was inevitable, you really freaked her out by getting all upset about it.
#4 there may be a possibility that she was never attracted to you in the first place. she may have been nice to you and friend zoned you right away.
you will find that it wouldnt have mattered wether she had a bf or not. if she was attracted to you, she would have hooked up with you. would you have felt any differently if you hooked up with not knowing she had a bf? i think not. the other point i want to make is about your interactions with her. were you just talking about boring logical stuff, or were your conversations going sexual at any point? my guess is that there wasnt, and it was just mostly fluff talk. i also think that you showed too much interest too fast and strated acting differently, that freaked her out.