Surely you're exaggerating. You've never heard of Etsy, okcupid, PlentyofFish, StubHub, or CouchSurfing? CouchSurfing in particular is mentioned very often alongside AirBnB, and okcupid has had at least one front page story on HN this year (namely their acquisition.)
I might be an atypical user (?), but none of those sites appear to do the two things I and everyone else I know use Craigslist for: 1) buy/sell used goods; and 2) find long-term apartment rentals.
For #1, eBay seems to be the only real competitor, and does non-shippable goods poorly (Ebay Classifieds has never really taken off). For #2, I don't really know of any (AirBnB and Couchsurfing are good for trips, but not really for finding somewhere to live).
I agree with you that none of these sites actually tackle the main uses of craigslist. I was merely questioning your assertion that you'd never heard of any of them :).
The apartment spam on Craigslist just means that it's harder to find an apartment, not that there is some other site to use.