An interesting site - Introduction to Genre Theory - although it's chiefly about the audience construction of T.V. genres has this to say:
Constructing the audience
Genres can be seen as involved in the construction of their readers. John Fiske sees genre as 'a means of constructing both the audience and the reading subject' (Fiske 1987, 114). Christine Gledhill argues that different genres 'produce different positionings of the subject... Genre specification can therefore be traced in the different functions of subjectivity each produces, and in their different modes of addressing the spectator' (Gledhill 1985, 64). And Steve Neale argues in relation to cinema that genre contributes to the regulation of desire, memory and expectation (Neale 1980, 55).
When I post to my blog here, or my personal blog in ELGG, I try to ensure that my tone is casual and I don't use too much theoretical terminology. (Today is an exception, or a new trend. I haven't decided yet;->) I do, however try to keep my spelling and grammar correct and carefully gauge how much personal information I include. I might get a bit more theoretical in the ELGG Pedagogical Impact blog and a little more impersonal. All these writing behaviors have emerged for me from reading other educationally-oriented blogs.
However, when I lurk in MySpace I see a very different blogging style and tone. It's much more casual about correctness, and much more revealing of personal information and narratives. Interestingly, it's also professional in its own way, as bloggers post information about performances and other things they wish to find an audience for. I see it as a different genre of blogging, a more social and youth oriented one.
So my current take on blogging is that it's a collection of related but distinct genres, evolving and taking firmer shape as more people read and write blogs and enact their expectations.