"How do you count users for an open source application that does
not require registrations? Downloads aren't accurate, since
downloads include the same users that could have updated and/or
downloaded revisions.
"What Mozilla does is they use their Application Update Service
(AUS) -- the service whereby Firefox automatically calls home once
a day looking for updates as a method. The number of users that hit
AUS on a daily basis are called the Active Daily Users. Dotzler
notes that there are 90 million active daily users. As part of a
mathematical computation, Mozilla multiplies the number of daily
users by 3 (hence the 270 million) to try and gauge how many total
users they have -- since not all user are active every day (or so
the theorem goes)."
