"So you wouldn't think it rude if your kid stopped, for example, taking your phone calls without explanation or warning? I guess I would think that quite rude."
No, I wouldn't, presuming as I stated that my kid was staying in touch by some other means. I might ask why my kid did that, but I wouldn't consider the action rude and I'd accept any reasoning given, including being told not to ask about the reason.
"And also -- isn't FB more than "just a method of interacting"? Isn't FB also a way to share photographs, details of your life, etc?"
I can and do interact in all of those ways with people in my life by other means already. Facebook is just a method for doing any/all of them, but you can do that stuff by other means. More importantly, the "child" is a college-age adult, and so has every right to choose the means of communication that she prefers. You didn't say whether this DD communicates with her mother by any other means so I might be presumptuous to assume that she's still in contact, but you did say they were close and so I'd find it really odd if she cut off contact with her mother entirely, and if that's the case then I'd have to wonder what else there is to the story so I still couldn't necessarily label the daughter rude.
"IN the example given in this thread - college aged daughter has a close relationship with her mother, but cuts her out of Facebook without explanation - is one I would say that falls firmly in the rude camp. I can think of several reasons why a college aged girl, newly away from home, might not want to be Facebook friends with her mother (both benign ones and troubling ones), but she owes her mother an explanation before cutting her off."
I disagree. Firstly, I find that one big problem here is that people seem to be forgetting that the DD is an adult. I'm very close to my mother but I wouldn't really want her to share in the discussions I had with my friends when I was in college, for a multitude of reasons. As an adult, the DD has every right to filter her Facebook friends down to what she wants, she doesn't need any better reason than "I want it that way", and she's not rude for using Facebook to her preference. Secondly, on the basis of my comment to Cami above, the rationale that defriending someone on Facebook must equate to cutting that person off entirely is factually inaccurate. Facebook is one of a very large number of methods of interacting, and like any other given method some people use it and some don't, and people use it for some things and not others. That doesn't make defriending someone (even without explanation) de facto rude.