Excuses to not allow a Gf/Bf on your Facebook. Any good ones?

I have a gf who wants me to add her to my facebook, but am trying to find excuses not to as I don't want my other gf to see her, and vice versa.

a) If you want to be seriously serial with your girl friends then bin all social networking sites (including TV.com), mobile phones, e-mail accounts and digital cameras. Also clean your hairbrush after every visit. Make sure you always have the same number of condoms in your bag.

b) Don't do Facebook or Twitter. If somebody asks why, say owned by the CIA.

c) take care - read this Woman Stabs Boyfriend Over Facebook Page