How to-do apps make money

I am working on a subject study: how to-do apps make money.

There are many popular to-do apps such as Wunderlist, TickTick, Todoist and Any.do. And there is no doubt that these to-do apps are helpful to boost our productivity.

By studying their revenue model, I can get the following conclusion:
1. Offer premium service
2. Provide enterprise edition for teams/business

For users like me, it is enough to keep a free account. And if a to-do app force me to upgrade to premium account, I will...

How to-do apps make money

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