The following chart is taken from the Mozilla Addon website on 10th of August.
It shows on the one hand the number of daily downloads (blue line) and on the other hand the number of active daily users (red line).
For me is the red line the more interesting one, because it shows how the user base grows and it rapidly does: Only in the last 8 weeks 8,368 additional users have decided to use EPUBReader. In sum EPUBReader was used on 9th of August by 30,210 users in 23 languages, so in my opinion the numbers are not that bad :).
If you wonder how the number of daily users is counted: every day a user starts Firefox, Firefox sends for each installed and active addon a request to the Mozilla site to check if updates are available, each update request is counted as one active user. So this number does just mean that the user has EPUBReader installed and it's active, it does not mean, that he really uses it. But I think, that people who don't like an addon, will deinstall/deactivate it earlier or later.
Let's see, what next day numbers will show, I'm very curious.