One can imagine them dictating, if speech recognition software is sufficiently good. This doesn't work for code (or, perhaps, it doesn't work for the programming languages we have now---a programming language with syntax designed to be spoken, not typed, would be very interesting) and it seems it doesn't work for writing fiction, as I think I recall Charlie Stross discovering (can't find the blog posts now, though
http://www.antipope.org/charlie/blog-static/2012/03/a-writin... discusses the problem). For day-to-day communication, though, the stylistic differences between dictated and typed prose are probably not important.
Moreover, this might yield a small efficiency win over both text communication as is (email, IM) and the telephone: dictated text would be as fast to produce as spoken communication (I can talk much faster than I can type, and I get the feeling that's true in general), while remaining as fast to read and as easy to edit as text.
I'm not sure this win would be very big, though; for me, at least, the bulk of the time I spend on writing is spent figuring out how to phrase things. Jackalope's comment, though, indicates that people are starting to see noticeable improvements in efficiency---the "how did I ever live without this" feeling. Perhaps they find speech more natural than typing, and spend less time agonizing over how to express themselves?
This scenario depresses me somewhat, as I have an inordinate and perhaps irrational fondness for slower, more considered forms of expression. I find my prose is more dense and pointed when I hand-write than when I type, which I prefer (though my superabundant laziness combined with the ease of editing on a computer mean that I almost always type).