Maybe a moderator would have easier access to these kinds of stats, but I'd be interested in knowing when the 1 millionth, 2 millionth, and 3 millionth posts also happened (month, day, year).
Would show how the popularity of the site has grown over time, as to how the length of time between 1 mil and 2 mil, and between 2 mil and 3 mil, and between 3 mil and 4 mil has shortened.