The book describes gamification, virality and retention in the freemium educational online platform. Relationships between virality and retention parameters as measurable metrics are calculated and discussed using real examples. The F-growth factor, which combines both virality and retention, is proposed as the metrics of the overall freemium system performance in terms of the user base growth. This approach can be tested using a small number of users to assess the system potential performance. If the F-growth factor is less than one, the product needs further development. If the F-growth factor it is greater than one, the system retains existing and attracts new users, thus a large scale market launch can be successful. The book also describes the experiments to attract active online system users to the partner program. The objective is to grow the number of users by involving existing system users in viral mechanics. Several examples of user motivation are given, along with the specific interface implementations and viral mechanics. Viral K-factor was used as the metrics for the resulting system growth assessment. Specific examples show both positive and negative outcomes. Growth of the target system parameters is discussed. The behavior of users as a result of introducing monetization. Monetization resulted in alternative system growth mechanisms, causing viral increase in the number of users. Given several options, the users choose the most advantageous and simple ones for them. Viral growth and monetization can be both competing and complementary mechanisms for the system growth.