Clay Shirky gives a short explanation of the Power Law and its application to the number of readers each weblog attracts. The Power Law basically posits that distribution in certain kinds of networks -- whether weblog or wealth -- is weighted in such a way that a disproportionate share of resources -- readers or dollars -- are allocated to the top performers in a group. In other words, the most popular weblogs get most of the readers, just as a few rich people control most of the wealth.
I came across the Power Law upon reading the unfortunate news that BlogShares has closed.