Hey there, I know this is not a nice situation to be in at all, but i think you need to talk to your girlfriend about this as soon as possible. When you talk to her just be as honest as possible with her. This way you will find out exactly how she feels about you then you can plan on what your going to do about it.
How do you know that you are not the person she has a huge crush on? Perhaps you should just ask her to find out, you will never know unless you ask her. You never know you may be that person. Sometimes face book is not always reliable that could have been from a long time ago and maybe had not been updated in a while. She surely would not have gone out with you, unless she had some feelings for you. You even said yourself that she seemed excited to go out with you, that seems like a good start.
I know you like her alot, but if things don't work out in your favour that is better than being strung along. You say you two get on really well. You could still stay friends but you have to accept that may know be going out with each other, It is better to stay friends than have nothing at all, right? You never know what could happen in the future, there could be a potential relationship. If it does not work out you will have to respect her decision. Least if she tells you now then it would not hurt as much as it would if you where going out for months. But to find this out you need to have a word with her.
Good Luck!