Letgo is the second fastest growing smartphone app in the United States, according to ComScore. Alec Oxenford, the company's co-founder, says the app originally used advertising for user acquisition but is now shifting focus to creating a stable and growing community to drive growth further. Letgo helps people find local buyers of goods with just the snap of a camera. Since it launched two years ago, it has already reached more than 75 million downloads. Oxenford says the company is exploring new revenue streams, including a premium model.