I've looked through all the comments here and while most of the answers seem to be along the critical mass variety/network effects/market reach, I think the better answer is branding. Specifically, Craig Newmark has earned my trust, and that of millions of others for no other reason than he's a pretty good dude. If you're going to beat Craigslist -- Craig Newmark -- at Craig's game, you're have to do things significantly better than they do. Not marginally better, but significantly better. And certainly there are problems with the level of spam in Craigslist, but those are solvable problems, given enough time. And the problem with Kijiji, which may have a better technical solution, is that it's run by Ebay, which once had my trust, but has since lost it.

Frankly, I wouldn't buy a ham sandwich from Meg Whitman.

As an aside, I'm listening outside my window right now, and am overhearing a couple of busboys at the nextdoor restaurant speaking in Spanish about how to find rooms for rent on Craigslist.

-- Fred