I have Installous, so that could make this possible. However you need to jailbreak (which often involves Cydia) in order to even get access to Installous, so how about the other 84%?

(PS: I only use Installous if I need to trial an expensive app - I will buy the app if it does what I need :))

One would not expect it to be at the average, because this information is going to have come from some popular app, and there will be high correlation between "people who have never used even a single app: they just wanted a phone" and "people who have never considered jailbreaking: they just wanted a phone"; this is even more the case once you consider that it might not be an app that "virtually everyone has": it might be an app that is either correlated with skill (you are slightly more likely to have the app, even controlling for people who have apps at all, if you are the kind of person who really cares) or negatively correlated with age (you are more likely to have the app if you are younger, which correlates better with the demographic of people who jailbreak).

That's a good question, I was wondering the same thing?

The question "how many users do you have" is impossible to answer, as all I can ever demonstrate is "X users used Cydia in the last Y period". As for your second question, the information I have for your average Cydia user (one that is not actively paying me money, in which case I obviously have tons of information) is purposely highly limited: I certainly do not have, for example, the "names" of devices that was included in these dumps from AntiSec (which often discloses the name of the user), or any of the other personal details that are claimed to be in the original file.