The 2 4 million is from site mods.
I arrived at the 6-800,000 users number based on taking how many alts the average user has (suspected) and essentially just doing percentages until I felt I had a good approx.
I could be wildly wrong though, this was just my best guesswork based on my experience here. But it's still most likely less than 1 million active users within the last year.