BookBub is offering discounts on eBooks and is basically Groupon for eBooks. In the first 1.5-2 years the company grew to about 2 million users without any external capital. Bookbub's business model is to charge authors for advertising in their email, but since at the beginning there were not really a lot of users, BookBub had to have a really cheap channel to get them. And since each user is basically not really worth anything directly, because there is no real buying conversion or similar, it would be really interesting to hear your opinion.
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