Even if they kept the privacy settings intact, I think my point would still be true because of basic network effects.

In the beginning, only other college students were on. Now that FB has spread out to the world at large, my FB social network is pretty close to my actual social network. A friend of mine has account, but hardly uses it pointing out, "What am I going to say on Facebook? I'm friends with my mother-in-law." I'm not as restricted a user, but I still don't say much.