Disclaimer: Before responding with any kind of official answer on behalf of the Stack Overflow Careers service here, I thought I would let some time pass to see if/how the community would respond to this query first. However, given that a full 3 days have already passed and there has not been a significant amount of traffic on this question, now seemed like as good of a time as any to offer perspective on the topic. I apologize in advance to the extent that this answer sounds like an advertisement for the Stack Overflow Careers Services. (ie: Obviously I have a vested interest in promoting our services).
Our Answer: While anything could potentially happen with a specific listing, the most common result is a nice set of generally well qualified applicants to each listing. In fact, a key benefit of the Stack Overflow Careers service is that a majority of the traffic to the site derives from Stack Overflow itself (Server Fault too).....generally a much much more well qualified batch of people than "the internets" at large.
Quoting one of the FAQ questions from the Careers site itself, we kind of come at this from a slightly different angle:
Won’t I get more applications using one of the large job sites?
You sure will! But what kind of applications? Do you really want to sift through hundreds or thousands of resumes from practically everyone with access to the Internet? When you advertise with us you’re reaching the cream of the crop. People who care about doing things right. People who are always looking to improve themselves and improve the way software is developed. You’ll get the kind of people who are just as interested in learning about business, management, and UI design as they are in learning about functional programming. You won’t get a huge number of resumes from unqualified people, but you will get great resumes — or you can have your money back.