Hey, whats up, man, you should be proud of your gf. If she doesent want to make public the relationship, then she is a good, normal and healthy girl. The kind of girls that are hard to find these days. Personally, I used to make my relationships public in facebook, till I met my new gf. And just like your gf, she doesent want to make relationships public. The difference is that I never changed my status, neither did her. And we are just great like that. I just told her one day "Hey, do we keep our relationship private on facebook? I think we should" And she answered: "Yes
I dont like to make it public". Thats all man, probably your gf is the kind of girl that wont make out like crazy in public too, and probably she is very polite. My gf is like that, and I like it. My ex gfs were nuts. Their lives depended on facebook and I think thats a bad sign. You also can realize that by the kind of pictures a girl uploads (if she appears alone in a lot of pictrues, in the bathroom, autopictures, etcetera.) Thats a bad sign, you dont want to have a relationship with one of those. Those are just to fuck. Try to live a normal life, a REAL life. Facebook sucks man, it invades your private life. So dont worry, your gf likes you a lot, my advice is that you leave your relationship as private.
Cheers! You found a good girl (thats a hard thing to do)