I will respond to reviewers, if we have a relationship, however casual it may be. The majority of the reviews I link to are found by my helpful Google spiders, which skitter around the Internet bringing me things without concern for my feelings. I tell them they're good and feed them lots of flies. Some reviews, however, come to me because the reviewer emails me directly to say "I reviewed your book." In those cases, I feel entirely justified in replying, privately, with "Wow, I'm glad you liked it," or "I'm sorry this wasn't your cup of tea, hopefully the next book will suit you better." Because we're in a private setting, interacting like people, as long as I'm polite, I'm okay.
The lines start to get a little blurry when newer forms of social media come into play. Like Twitter. If someone @'s me, they know I'm going to see their Tweet the next time I check my @replies. That's the culture of the system, which is built on the expectation of/hope for interaction. I don't answer every @reply, but I make an effort to read them all, and answer the majority. So am I responding to a review, or am I sticking to the dominant culture of the platform? What about on Facebook, where people tag to your profile? They know that doing so will send you a notification. Is that an invitation to interact, or is it a sad reality of the system?
Miss Manners never had to deal with being a polite, professional working author in the Internet Age. I think that's why she doesn't have any pointers for certain kinds of behavior, and why she never considers "get a baseball bat" to be the appropriate beginning to a polite response.
So where are the lines for you? What do you think is the boundary for "polite" authorial behavior—and from the other side, what's the boundary for behaving politely toward authors? Inquiring minds want to know.