I don't mean to be All Weinberger, All the Time, but he has written an important post on the future of the blogosphere.
blogs aren't even close to being a mainstream phenomenon the way email is. It'll happen. And here are some guesses (note: guesses) about what they'll look like when they do:
I am not quite sure that blogging will become mainstream in the way he suggests. There are a lot of people who "go online" but who do not have lives and personnas on line. They still see email and the Web as tools. Maybe that will change, but maybe it won't.
Also, a lot of people do not write fluently. Just as with newsgroups, we may see a lot of "lurkers" who do not wish to participate actively in the conversation.
I do not think that any of my daughters would enjoy blogging. And the time they spend on chat seems to *drop* when they get to college, for the same reason they spend less time giggling on the phone.
So one scenario is that blogging increases, but approaches an asymptote.
I think that David's points hold regardless, but my opinion would be that the probability of blogging becoming as mainstream as email is much less than 50 percent.