The game was created to be casual, open to anyone to play, and use some really memorable characters that would resonate with anyone who picked it up. They accomplished those goals -- the game has found a surprising following around the world, and almost everyone who owns an iPhone recognizes the birds and pigs characters. A US$4.99 iPad version has also taken off, and Rovio plans to bring the app to lots of other mobile devices as well. The growth has been entirely organic, too -- they haven't spent a single cent on advertising the game, instead relying completely on word-of-mouth and Apple's own promotional tools.
And while their success is already quite widespread, the two say their goals are even bigger than what they've accomplished so far: they hope to eventually see 100 million paid downloads. Definitely possible -- it's interesting to see how iPhone-created brands are growing so quickly.