Orin Kerr found that law review citations to our friends at the Volokh Conspiracy have been increasing significantly over the years. Using the same methodology (citations to balkin.blogspot.com in Westlaw JLR database limited to each year) I discovered the same thing is true of Balkinization. In 2003 we received 1 cite; in 2004 3 cites; in 2005 14 cites; in 2006 36 cites; and in 2007 49 cites. As Orin reminds us, some law journals have not yet published all their 2007 issues, so the final number for 2007 may be slightly higher.
These results suggest that blogging has become a more widespread and accepted practice in the legal academy. It's important to remember that people cite for many different reasons: to give credit for ideas, to criticize ideas, and as (persuasive) authority. My guess is that most of these citations fall into the first two categories, but that is true to many citations for law review articles as well.
On the Law Librarian Blog, Joe Hodnicki provides more evidence for this.
This built-in undercount does not diminish from the fact that the below statistics do give a sense of the magnitude of the growth rate of blog citations. According to this estimate, blog citations in law reviews and court opinions have grown from about 70 in 2004 to over 500 in 2007 (and still counting since many law reviews have not completed their 2007 publishing cycle). I believe it is fair to say that for 2005 and 2006 blog citations probably grew exponentially on a document count basis, doubling each year.
It is unlikely, however, that any final count for 2007 will show a similar rate of growth. If the case, would this mean that blogs are "on the decline." Doubtful. It would simply mean that the blogging phenomenon is maturing. As with other forms of publication, with age comes acceptance and recognition of place within the structure of legal literature.