I believe that is strongly tied to why they have become so popular so fast. You can go into the game, pick it up, and become fairly good quickly. If you continu

I believe that is strongly tied to why they have become so popular so fast. You can go into the game, pick it up, and become fairly good quickly. If you continue on with it, you slowly get better as time goes on. You then get a friend to join, and they have to start at the beginning. They could spend the days/weeks it took for you to get that far, or they can spend $5 and get there in a tiny fraction of the time. To generalize it, its easy to get a handful of people to do something. If you offer it to them for free, there are plenty of people who will go out of their way to advance as far as possible. Once those people start getting friends involved, the friends will want to get that far too. By offering them a quick advancement in exchange for money, you can then make a tidy profit.