One idea that comes to mind re your question of what happens to the
individual speaker and persona in a social theory of writing: Bakhtin
posits the notion of individuals interacting with (negotiating,
reinventing, putting their own spin on) a society's existing genres.
Here the individual does appear to retain a certain autonomy: there's a
reciprocity between individual intention and expression, on the one
hand, and conventionalized social uses of language, on the other.