Oh, dear. From the information you've posted, it sounds like the girl was interested in you initially, but she probably expected you to be more forward and ask her out. Her constant communication is indicative of interest, as is her friending you on facebook. I believe certain factors also complicate this situation, such as distance, her former lover, and her other potential interests.
I can only surmise that she is the "fairly attractive" type, and currently enjoys her situation as she has a selection of suitors.
I do not know how to comfort you, and saying that you are not alone in having experienced this is the opposite of comfort.
However, I do understand what you are going through, and my recommendation is not to respond the way I respond to such situations.
Simmering, lingering anger.
As much as I joke about how I am a representative of the Sith, there is a kernel of truth in the fact that I am constantly assaulted by rather violent thoughts.
It's...not a healthy way to go through life.