Today CL's sales are estimated to be ~$335m and doubled year-over-year (2)
(1) http://www.sfgate.com/business/article/EBay-buys-25-stake-in...
(2) http://www.wsj.com/articles/ebay-sells-craigslist-stake-back...
Craigslist is an insanely valuable source for lead generation in recruiting, and car sales.
I run a recruiting company and craigslist is the best source of applicants by volume and return on investment (cost ($) per applicant).
Having observed them closely, they are not ignorant to this fact.
When specific cities or sections grow more popular they begin charging fees.
Their biggest boon has been a recent change to charge car dealerships 5$ per post. Car dealers pay because only months earlier they were paying for black hat solutions to post for 1-2$ per post.
Yeah... the most lucrative 20% of the volume...
This is why you see Apple buy dozens of companies quietly without disclosure all the time. Because most purchases would not really change whether the average person invests, if for no other reason than the sheer size of Apple.