Assuming the level distribution of all users is similar to the level distribution of forum users, level 3 will probably have the biggest relative dropoff due to the free trial being over. You can see more about this distribution on the level distribution thread: What is the level distribution on here? - #341 by BIsTheAnswer
Aside from that the dropoff is relatively consistent. Disregarding the free levels the curve is nearly exponential, meaning about the same percentage of users stops each level.
I’d be interested in seeing the other stats too. I like seeing stats 