There are straightforward, well-used definitions of "user" for which I have 60, 1.5k, 2.9k, 50.0k, and "well over a hundred thousand" users. They're all equally true and none of them singly or in combination tell you a thing about the health of my business unless you're intimately acquainted with it.
It does? Says who? This is your definition of a user. That's fine, but it doesn't extend to being a common one.
Unless of course you delete all the accounts of people that have registered for your service but aren't using it as often as you'd like.
The question is what does "3 million people who use our web-based apps" mean.
A simple truth test: Are there "3 million people who use [their] web-based apps"? Absolutely not. Not under even the most remotely reasonable definition.
:-)
My patented uniques metric: cat access.log | awk -F " " '{ print $1 }' | sort | uniq | wc -l
cut -d ' ' -f 1