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Linux lacks any clear-cut system for determining which is the most popular or the best distribution, or which desktop environment is used more than others. That may be one of the major frustrations among Linux developers trying to spread the word about adopting the Linux desktop instead of Microsoft Windows or Apple's OS X.
Linux, like the countless denominations of religions, is a product of technology, ideology and endless options. Unlike its competitors, the Linux OS is not sold. It just is available to those who know about it.
In our connected world where most people use the Internet probably the best metric we could get would be which distros are accessing web sites. Ideally we could get those numbers for the most popular web sites and that would give a pretty good reflection on what is being used on the desktop. We do have those numbers from Wikimedia, which includes Wikipedia, the fifth most popular website:
I think that provides a really good indication of what platforms are being used in the real world. Sure, some users spoof their User Agent string, but I would guess that would be a statistally insignificant number that has an equal effect across desktops.
Obviously everyone doesn't use Wikipedia, and ideally we could get an even more accurate number if we could get similar statistics from Google and Yahoo.
Even so, I think these numbers are a lot closer to the true platform use than either download numbers or Distrowatch page-hits.