I'm also not sold on the idea of giving them what is on the tour and they will retain. After all, how many different pic sets do you use on the tour? I would be willing to bet it's not that high. So they can get through those in a pretty short period of time. Then what? I also don't think giving them a ton of other content is the key.
Updates are crucial, but I don't think that is the only thing. IMO, the price point is the biggest factor in retention. Find the right price point for the content you offer and the amount of updates you provide and you will have many happy PAYING customers.
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