Endomondo is a fitness app that strives to make working out social. You can publicly log your physical activities and challenge your friends to do the same. It is a way to provide positive peer pressure into mileage. This article states that research even showed a strong correlation between workout activity and the number of friends a user had.
The most important milestone this app has reached is that it is now profitable. The app launched five years ago and is seeing success. This can go to show, however, the long process an app takes to gain profitability. Even with one billion miles of activity tracked and an international user base, the process can be slow especially in a market that is flooded with fitness apps.