Well, not on blogs exactly, but internet communication in general. What he says definitely applies to blogs, though. The quote is in a footnote in this speech that Habermas gave at the 2006 annual convention of the International Communication Association.
Allow me in passing a remark on the Internet which counterbalances the seeming deficits that stem from the impersonal and asymmetrical character of broadcasting by reintroducing deliberative elements in electronic communication. The Internet has certainly reactivated the grass-roots of an egalitarian public of writers and readers. However, computer- mediated communication in the Web can claim unequivocal democratic merits only for a special context: it can undermine the censorship of authoritarian regimes who try to control and repress public opinion. In the context of liberal regimes, however, the online debates of web users tend instead to lead to the fragmentation of large mass audiences into a huge number of isolated issue publics. The rise of millions of fragmented chat-rooms across the world endangers only political communication within established public spheres, when news groups crystallize around the focal points of print media, e.g., national new-papers and magazines, which are the pillars of national public spheres.14 (A nice indicator for the critical function of such a parasitical role of online communication is the bill for € 2088.00 which the anchor of Bildblog.de recently sent to the director of Bild.T-Online for “services”: the bloggers claimed they improved the work of the editorial staff of the Bildzeitung with useful criticisms and corrections.)
You don’t have to look hard at the blogosophere to see just this “fragmentation of large mass audiences into a huge number of isolated issue publics.”
Anyway, I know this is two Habermas posts inside of a week, but I’ve been reading pretty much everything even remotely associated with Frankfurt, lately, and this quote struck me as apt.