, I think our concept of privacy is different, but I think there's also a difference between our concept and how our technologies are shaping privacy. Think how people think about their "private" posts to Facebook or Google+, for example. Both of the technologies chip away at the concept of privacy that most (or at least lots of) people have -- and they do so invisibly.
When Erasmus wrote to More, he no doubt realized his audience was much larger than the More household -- and that played a role in the way he framed his argument and his message. But we think we're communicating privately, when we should take the technologies into account and be suspicious of that concept of privacy. (Think SEO for "personal" correspondence! Google fashions ad pitches to you based on your email.)
The email that Susan Sonntag wrote is interesting to me because it makes me wonder about the dissemination of ideas with emails. It's so easy to forward an email, and I can think of emails and communications that have exploded in influence because they've been easy to share. All good biographers will consider a communication as an interaction
-- and for letters it's been an easy task. My view is that for emails it's a much more difficult task to weigh and consider.
(Pace, Walter Benjamin: The Work of a Letter in an Age of Internet Reproducibility