There is currently no way to reach them all simultaneously. They all might visit different Web sites at different times of the day. Even if they browse the same Web sites, they might use them differently. The Internet as a commercial medium might have a high penetration, and it might even approach TV's ubiquity one day, but its consumption is more closely related to that of magazines. Like magazines, Internet communities and the people who compose them tend to organize around common interests and lifestyles.
Content is self-selected.
My point is that each Internet user makes use of the Internet in a unique way. In my mind, what makes the World Wide Web a web is not the fact that it exists as a series of linked sites, but that it is an amalgamation of nonexclusive communities. Fan Club newsletters every week. The Holy Grail is reaching the people who can identify with your brand in the appropriate place and manner. Coca-Cola shouldn't be blasting the Internet with Coke ads in an attempt to mirror its TV strategy.