Although I'd heard of Facebook as early as it was born 3 or 4 years ago in the States and got very popular among high school and undergraduate students, I never used it. I didn't have any undergraduate friends in the university, and I didn't teach so I didn't have to interact with them. None of my friends in the graduate school was using it. It's natural that I didn't need it to communicate with people.
However, things changed. A friend I met in Istanbul sent me a Facebook invitation months after we parted. I was not surprised, because she is an American. She might have used it since she was in college. Another few months passed by, and another invitation came. This time, I was surprised because it came from a Chinese colleague. I thought, "What? Asian students get to know this site and start to join it?" It arouse my curiosity, so I registered an account.
I found later that a few months ago Facebook started to welcome non-academic users to register. No wonder a lot of non-American-college students joined in. But I was shocked when I saw many British colleagues of mine on it. I thought it's just undergraduates' networking toy, but now it looks like those undergraduates have grown up and brought the habit into graduate schools. I was always concerned about not having enough time to social with my British colleagues, and now my problem is solved. I think the private and informal interaction on Facebook helps us, the extremely busy students who seldom have tea together, communicate better.