A few suggestions: - find a way for me to challenge my friends so you get some virality and the game is more meaningful - offer multiple skill levels. Start easy so you can get teachers to recommend it in their classes - let us "boast" if we got a question right or a streak of questions right
If one could select an easier level, I'd push some nephews / nieces to that site. Then, I guess a bit of "challenge friends" could aid in them actually using it to improve their math skills.
I think there might be a market for GRE-level math questions. If your site loads quickly and is generally snappy, test takers could stop by for 5 minute lunch sessions.
You could also start with easy questions and have it slowly adapt to harder/easier questions as you get questions right or wrong (maybe accounting for speed as well).