Curiosity killed that cat but I am still curious about some definitions here. In the newsletter there are this section with Top Learners This Month. What is the definition for this month? Is that the previous month or is it from the 15:th to the 14:th? Further, This lists only people that do have an open profile or? So in reality there are probably others with higher stats that not is shown?

Finally, the stats today compared to last year same time are impressing:
2011:
Hours spent: 6,462 // Items studied: 5,041,304 // Characters learned: 68,599 // Retention: 90.39%
2012:
Hours spent: 12,742 // Items studied: 8,907,429 // Characters learned: 199,786 // Retention: 87.96%

Retention have been steadily going down. Is retention here based on the last months stats or the overall stats? If I look at my own stats I can see the same tendency, the more new words i add the less retention i have. So the logical explanation is probably that users have a larger number of items to review, right?

It's either 16th to 15th or 15th to 14th, depending on if I generate the statistics the day before or the day of.

Yes, people with private accounts set don't show up in the top learners, and there are some that study more than the public top learners.

Yup, site usage is way up from last year! The average retention rate has been down about 2% since the iOS app came out. Whether this is due to a change in the way people study using the iOS app, or because many new users are here and learning lots of new words, or because many old users came back and had to deal with many forgotten words, or something else, I'm not sure. But 88% is still pretty good; SuperMemo suggests that even lower retention rates can be more efficient: http://www.supermemo.com/help/fi.htm#FAQ