In this entry, when I talk to blogs, they specifically refer to personal blogs, not general purpose blogs.
Many of my friends (mutual and unilateral) blog. For several reasons. I suspect many of them do it with the same reasons as many others indulge in exhibitionism, what one of my friends/colleagues called "public prostitution". I don't mean to be so scathing however. Exhibitionism is the extreme form some people take emotional release to. There is some justice to the theory of catharsis, letting go of the self, to achieve emotional balance, by letting the floodgates open. This has been proved to be a very useful way of clearing the mind of useless muck and refocusing it on the important. People used diaries for this purpose earlier.
However, what would be interesting to analyse is whether people in the past would have liked to show their diaries to others. I don't know. Assuming that very few people showed their diaries to others, we are led to conclude that there are many more private blogs that exist on the internet than public blogs. In fact, more with a certain factor.