Here are some more data points about calibre's popularity:
Number of unique IP addresses that have used the calibre Fetch News feature:
Current calibre-ebook.com website ranking: http://www.alexa.com/siteinfo/calibre-ebook.com
Top 5000 of *all* websites in the US and top 7000 in the world. And this is with a website that employs no traffic generation tricks, SEO, advertising, whatever.
According to google analytics, there were 2 million unique visitors to calibre-ebook.com in the last week. Of which 60% were new visitors and 40% were returning visitors. Unfortunately, I can't create a public link for this, so you will have to take my word for it.
Assume that 40% returning visitors implies at least 30% active users. That would put calibre's active user number somewhere between 4 and 6 million active users (from a total of 13.7 million).
I doubt very much that there are 4 *million* computer geeks/nerds in the world that are also avid e-book readers.