Google is a classic example. Although all my friends were using Google when it was hosted at Stanford, when they got some money I still wondered, "but who thinks the world needs another search engine?"
The reason it became popular was pretty simple. It was fast, clean, single purpose and had no advertising. It was the complete opposite of Altavista and Yahoo at the time. And as we've seen countless times there is always room in a market for simple, focused products.
That means they needed a lot of startup capital to get started. No adds no income. Give the heroin out for free until people get addicted.