Developing a rapport with the people you recommend can increase retention & even bring in more recommendations from their own contacts. The market is flooded with Refer & Earn apps, all of bigboost free slotswhich provide different incentives and rewards. Among the best apps to think about are: 1. Uber: The ride-sharing service provides bonuses for referring new users who sign up through referral links, as well as for the referrer. 2. . Airbnb: Travel credits are awarded for recommendations, and these credits can be applied to subsequent reservations on the hotel booking website.