f your game has a good viral ratio of 0.5, that is, each person invites 0.5 other people, then we can model the equation as 1 / (1 - 0.5) = 2. So for every person we buy, we get 2 kicked out (2 for the price of 1). So, to hit 0.5M MAUs we need to buy approx. 0.5M / 2 = 250,000 users.