Mac users

A recent study by WebSide Story showed that only 3% of web users are on Macs. This is suspiciously low for a platform with at least 10% of the installed base!
The problem isn't the numbers, but how they were derived. WebSide Story used HitBOX Tracker. As Ben Wilson of MacCentral notes, "It should be emphasized that the HitBOX service only provides a viewer for Windows, meaning not many Mac-specific sites are likely to use the service."
Simply stated: if you use a platform related measuring tool, results will be skewed toward that platform.

A survey of visitors to Low End Mac shows very different results from those created by
58.5% use the Mac OS (not 2.7%), and only 36.6% use Windows (not 94.5%)
63.8% browse with Netscape, 34.3% with Internet Explorer (not 68.7%), and 0.7% use iCab
Admittedly, these numbers show a bias toward the Mac side, just as the WebSide statistics show a pro-Microsoft bias in both OS and browser.
The truth is, we'll never have precise figures comparing Mac OS, Linux, Windows, and other market share or installed base. And any survey of the internet will be biased by the sites and software used -- sometimes just a little, but sometimes a great deal.
In reality, Mac users comprise somewhere over 10% of the installed base, but probably less than 15%. Based on that and the ease of connecting all but the oldest Macs to the internet, we can estimate that Mac users are at least as likely to be on the internet as Windows users, so at least 10% (and perhaps as high as 20%) of internet users are Mac users.
Anyone purporting to offer statistics on something as vague as the installed base or use of the internet with three or four digits of precision is being unrealistic. While the numbers may mathematically provide a figure of great precision, the nature of the study leaves a margin of error of several percentage points -- just like those public opinion polls in the press and on TV.
We can safely say that Windows users outnumber Mac users in the home, in the office, and on the internet. We can estimate there are about seven times as many Windows users as Mac users.
Beyond that, we have to remember that there is a great margin of error in these surveys. I would be a fool to extrapolate from visitors to my site and claim that almost 60% of all computers on the internet are Macs, despite the numbers which could be construed to "prove" it.