It won’t be your father’s Internet for much longer.

Jakob Nielsen, a consultant who’s spent more than a decade studying how people use computers and software, cautions the Web needs to become more simple by 2015 when another billion people begin surfing.

“Some time this year, we quietly passed a dramatic milestone: the one billionth user went online,” Nielsen reported. Absent a central registry of Web users, he suggested, “Statistically, we’re likely talking about a 24-year-old woman in Shanghai.”

In an essay about the import of 1 billion new Web users in the next 10 years, Nielsen bluntly warned Web-commerce business operators: “Users are not like you.”

Today’s Internet is diverse, “and has moved far beyond the elite in Silicon Valley and other global technology hubs. Selling to the 200 million early adopters was easy. The 800 million mainstream users who are now starting to shop need much smoother sites; the next billion will require even higher usability levels.”

Two-thirds of Internet revenues will come from countries other than the United States, he says.