Their secret is they did not try to grow at an insane rate. They have 50 employees and over $650 million in revenue. The revenue per employee is over $13 million.
The reason the giants are giant is because they are driven to become giant by people and investors who want to become billionaires.
Craigslist has indeed done very, very well. They've maintained a useful service, ad-free, for decades. Made a ton of money doing it, too.