Facebook? Just kidding.
I've had quite a few FBs in my lifetime. Obviously, they just fill a temporary need. Most FBs, for me at least, last 2-3 months. But I do have one, that I can call on wherever and whenever, for over a year and a half. Usually, my FB relationships end after I get into a committed relationship or they seek one from me and I decide that I don't want to go into that direction with them.
Really though, FBs are all temporary and they WILL end. When they do, never blame yourself. Eventually, she's going to want something more from you or find someone else from whom she will want something more from. It's at that point that you find out if you can move on or you realize that you've actually been more attached to her than you originally thought you were.