Google Inc. said today that it bought Instagram rival Nik Software, which makes the award-winning photo editing application Snapseed, for an undisclosed amount. Google and Facebook Inc. are locked in a battle for social network followers that has increasingly shifted to mobile applications, such as photo editing. Though not as famous as Instagram, which is available for free on Apple Inc.’s mobile devices, Snapseed has won a following for its editing prowess among photographers, despite a $4.99 price tag. Nik Software says Snapseed has more than 9 million users, while Instagram says it has more than 100 million.