This is an example of what I call "making a big deal out of nothing."
Hope that doesn't sound too rude, but really: it's NOT a big deal. I don't add any of the girls I'm dating to Facebook at all. If they ask about it, I give them one of 3 answers:
ANSWER #1: I've seen too many relationships go bad as a result of facebook post misinterpretations, so I make it a point not to add anyone I'm dating
ANSWER #2: I feel as though you (talking to the girl) have the right to keep some things private just as I do. I don't feel like whatever you're posting on your wall is my business and vice-versa.
ANSWER #3: I'm already friends with you in REAL life; no need for you to become a fake friend of mine on Facebook.
If they have a problem with any one of these answers, I could care less. Foot in the ground - she ain't gotta like it, but she'll at least respect it.