Even after possibly using some sort of small bot network, the total app downloads had to be relatively minor at the beginning of January. It was still going completely unnoticed on the App Store. (I’d suggest probably less than 10,000 iOS devices had Flappy Bird on them before January 9th.)
January 22nd Dong Nguyen was probably extremely excited about the couple months worth of revenue his marketing experiment had pulled off. With the recurring in-game ads and 800 reviews in a single day, Flappy Bird was beyond a success. Mission accomplished.
February 1st Dong Nguyen, on the other hand, must have questioned if the world had lost its mind.
On February 1st, reviews exploded to 800 in a single hour. 6,500 iTunes App Store reviews in a single day. February 1st is the day Dong Nguyen woke up, stretched, checked email, checked Twitter, checked iTunes, and witnessed millions of downloads happening.
An interesting look at the somewhat suspicious chronology of Flappy Bird.