Very interesting, I completely screwed up the early marketing part, the game page presentation and couldn't get SFX in the game... Without rate exchange I'd probably have very few views so I've taken goo notes on that.
For the difficulty curve, what seemed to work for me is to introduce new concepts or mechanics over a few levels so that first the player sees the new thing and has an easy level to play with, then a level slightly harder where they have to expand on how to use the mechanic then some levels where they are on their own.
But I guess you made the difficulty right if you scored that high ;-)