The data is taken from the Mozilla Addon website on 21th of September.
The graphic shows the number of active daily users. As you can see, the user base grows pretty fast: Only in the last 4 weeks more than 22,000 additional users have decided to use EPUBReader, that equals 700 new users per day. In sum EPUBReader was used on 20th of September by 201,320 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.