Angry Birds developer Rovio turned down a US$2.25 billion acquisition offer from games developer Zynga last summer, reports the New York Times. Sources said Finland-based Rovio walked away from a deal, which included cash and stock. Zynga, which made its name building social games such as FarmVille, is currently heading towards an IPO and had more than 11.1 million daily active users for its mobile games in October, according to a US Securities and Exchange Commission filing. Rovio meanwhile has said it's aiming for more than 1 billion Angry Birds fans as it continues its global expansion.