  The paper describes gamification, virality and retention in the freemium
educational online platform with 40,000 users as an example. Relationships
between virality and retention parameters as measurable metrics are calculated
and discussed using real examples. Virality and monetization can be both
competing and complementary mechanisms for the system growth. The K-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 K-growth factor is less than one, the product
needs further development. If the K-growth factor it is greater than one, the
system retains existing and attracts new users, thus a large scale market
launch can be successful.
