Sounds like an very conservative forecast. When the 2000 figures are in, they will probaly show around 410-415 million on line as of Dec 31, 2000. For this to become 1 billion by 2005 requires a mere 20 percent annual growth. But the historical growth has been around 75 percent per year. The authors have not explained why they believe in a sudden slowdown.
I can think of one reason only. Market saturation in North America and nortwestern Europe. That would explain why the global growth cannot continue at 75% per year until 2005, which would lead to an online population 6,8 billion - more than the number of people on the planet.
My guess is that the online population in 2005 will turn out to be at least twice as high as the forecast - 2 billion.