How long have you known her? That would help me determine how I would answer.
If you've know her less than a year she kind of falls into "neighbor" status. Someone you know or THINK you know, but really don't. They can be quite polite while in your life, but all of a sudden a moving van can show up and off they go...
Friendships take a few years to really build and even then we lose friends along the way as our lives and relationships, jobs, kids, circumstances change.
You are dying to know what's going on with her, but she doesnt want to share it with you.... she's got other things going on that are more important at this particular moment, things that she may know that you are not able to help her with so she's chosen not to share with you.
I'd let some "healing time" go by, and if you still miss her in 6 mos, try looking her up on FB again.
I'm an apaprtment manager, a lot of my tenants consider me their "friend" because I'm nice to them and listen to them. But I am NOT their friend, I would not invite them to my house, I would not ask them to meet me for dinner, however they invite me to stuff continuously.. THEY THINK I'm a friend, I am not. So, see what I mean? You are looking through a different filter than "your friend" did. She had different expectations of your relationship than you do.
Don't be hurt by it, it's just life and it happens.