Some externals that had a huge influence on Flappy Bird are Vine and then a couple of huge play along YouTubers (such as "pewdiepie"). I made the analogy before that Flappy Bird is the 2 girls 1 cup of games, and while I saved myself from ever seeing that video, it became famous in reaction videos. Exactly the same thing happened with Flappy Bird, first on Vine in early January, and then as the mega-YouTubers picked it up in late January. It was likely pewdiepie who yielded the enormous uptick at the end of January.