Well, at least we can try to have a rough estimate - in the end, it's more the variation and the scale, rather than the exact value, that is significant.
We have several potential sources of information that we can refine, if we all agree on these - that involves from developers to sysadmin and mirror admins.
1. we can count the number of download requests from the Web site - if we take as a habit to always route people to download from
http://mageia.org/downloads/ (and not to go in their FTP mirror instead, to which they would likely be redirected anyway) at some point, if should be somehow representative of most downloads. That says nothing on actual installation and usage rate however.
2. we could suggest mirror admins (that are ok with it) to provide download log (or give them a script to parse/compute their logs) so we can get the request numbers; not all mirrors will provide this, but that should not prevent from offering to do it; even if we have only a few mirrors in this game, we can correlate their numbers with above 1) numbers.
3. we can count requests from mgaapplet to the releases.mageia.org website; at some point, this can give a representative activity of online systems; that have a somehow accurate number would need some work about the refresh/update frequency to this releases server.
4. we can count requests to the mirrors list API on mirrors.mageia.org; but same as above, it needs some work about the frequency to get a somehow acceptable estimate.
5. other ideas?
All in all, this will need 2/3 people to gather and set this up in a working, consistent, approved manner and to publish this in our public dashboard. This relates to the privacy policy as well as we have to make obvious that no identifying info is/will be used in these processes and that the compiled results are to be public.
And, again, this won't provide a very accurate picture, but a rough estimate/scale, still interesting enough.