Dropbox

Dropbox had a genuinely useful product in a category most people didn't know they needed and couldn't find through search. Paid ads didn't pencil out: "cloud storage" wasn't a term regular people typed. Drew Houston launched in 2008 with a demo video aimed at Hacker News that shot the waiting list from about 5,000 to 75,000 overnight. Then came the bigger move, a referral mechanic that helped take Dropbox from roughly 100,000 users to 4 million in 15 months and collapsed acquisition costs.

For founders and operators, this is the cleanest case on building growth into the product instead of bolting marketing on top. The reward Dropbox offered wasn't cash, and the alignment is what made the loop self-reinforce. It sharpens the decision of whether there's something your customers already want more of that you could trade them for spreading the product, and exactly where in the journey to ask.