If your startup is building a consumer product, your product has to be viral. For consumer products, the average revenue per user tends to be relatively low, so you won’t be able to afford spending much on advertising or sales. It’s hard to slap a viral channel onto an existing product, and you’ll have to think about how to make viral growth happen right at the start. Consumer product startups have to bake a viral channel into their product from the get-go. They can’t merely glue it on later.