With the way that CPAN is currently set up there is no way for module authors to get any idea of how many people use their modules. Ok, if the module in question is DBI or LWP::Simple, then you might have some vague idea that a _lot_ of people are using your module, but most of us don't write modules that are that popular.
You might get the occasional email from someone asking how something works, or perhaps a bug or two raised in RT or (even more occasionally) an email saying "thanks for the great module" but generally you're working in a bit of a vacuum and you just keep pumping out releases in the hope that someone finds them useful.
Over the last couple of days, I've (accidently) discovered a good way to get a better feel for how many people are using your module. I released a couple of versions of Calendar::Simple that were broken in nasty ways. I realised what the first problem (in 1.15) was pretty quickly, which is why I released 1.16 very quickly afterwards. And then, within a few hours, I started getting reports of another (even bigger) problem in 1.16.
I didn't think that many people used Calendar::Simple. But now I know that there are at least a reasonable number of people who use it and get upset if it gets broken. Which is a good feeling.
And I've released version 1.17 which fixes all the reported problems.
Update: I spoke too soon!