Craigslist started with a lot of word of mouth marketing. It was the de facto classifieds site in the Bay Area before it opened for other cities. It started as and still is essentially a not-for-profit.
On a side note, Craigslist is hardly successful outside of the US. Even in London it's got no traction.
Craigslist is not a non-profit. They've had well over $100MM in revenue for several years running, and that is mostly split by Craig and Jim. The "non-profit" aspect of Craigslist is a myth that they play up.
On a side note, Craigslist is hardly successful outside of the US. Even in London it's got no traction.