Top smartphone apps are mostly available for free and hence no revenue measure is applicable. An app is popular based on its usage and hence, a smartphone measure suitable can be reach among all mobile users or the smartphone app penetration. Mobile statistics data sites in public domain analyze mainly US smartphone apps usage data. Hence the parameter chosen in reach of US smartphone apps for 2014.
Almost every photographer out there must have used this one at least once. Instagram is an online photo-sharing, video-sharing and social networking service that enables its users to take pictures and videos, apply digital filters to them, and share them on a variety of social networking services, such as Facebook, Twitter, Tumblr and Flickr.