You're right about Craigslist and maybe Zappos, but Facebook won, among other things, for being the first truly usable, non-hideous social network. And they keep creating new UX elements.

Amazon has so many remarkable UI inventions that they patented some (not that I agree with that).

eBay has pioneered a lot of UI innovation as well.

Usability and UX invention is in the core of this 3 companies.