How did Sahil get Gumroad to grow?
Sahil built a product that solved a problem and had a word of mouth component, enabling it to grow on its own. He says the key was to figure out the audience, and then to talk to them, to figure out how they were using the software, and if they were not, then why.
Growth-hacking only works when you've got your product and funnel figured out. Many successful startups start using "Collison Installation", doing things that don't scale, one customer at a time.