We conceive of the Internet as a medium of mass communication, and we analyze 2 behavioral features of its audience: size and duplication. Internet audiences are highly concentrated in the most popular Web sites, conforming to Pareto's Law. Duplication across pairs of Web sites is explained by the size o f the audiences unique to each site in the pair, conforming to Goodhardt's duplication of viewing law. In addition, similarities o f content or domain modestly increase duplication across Web sites. A 4-variable model explains over 80% o f the variance in duplication.