10,000 concurrent users within a day of release is massive success in today's application market. Engineering a system to handle an order of magnitude more traffic than you expect is sensible; engineering it to handle three orders of magnitude more than you expect would, virtually always, be ridiculously wasteful.
Also, Curiosity is not your average horizontally scalable web app, because all the users are continuously interacting with the same data set. Scaling this up to 2 million concurrent users would be a challenge even with time on your hands.
Very off topic, but what are your thoughts on this;
I built something alone for both the client (iOS with Cocos2d) and server (Ruby, MongoDb, Redis) that managed 3k users on release for a full day. The app was only for a day (Event app for an ad agency)
This took three weeks to develop. The client had 3d graphics and a custom map coded and drawn from scratch.
Also, Curiosity is not your average horizontally scalable web app, because all the users are continuously interacting with the same data set. Scaling this up to 2 million concurrent users would be a challenge even with time on your hands.