I think that in some fields people are more active in the internet. There seems to be some correlation between the most popular tags here and the number of blogs in the corresponding section in the mathematics/statistics blog wiki here:

http://wiki.henryfarrell.net/wiki/index.php/Mathematics/Statistics

I do not know the relative proportion of algebraic geometers to other mathematicians but it may be in some fields the internet is more useful than others and there is more activity of the people in the field.