You should use assessment tasks that incorporate subversion -
One of the worst things you can do is mandate posting on particular topics with
particularly rigid frequency… you’ll over-assess & kill off exactly what
blogs are good for: personal expression & exploration. By all means say that
you’re expecting a post a week… or ever more, but let people approach this in
ways that fit them and set tasks that allow for deviation and subversion. Never, ever, mention number
of words!