Is there critical mass? I would say, in major tech/development areas, or for national-footprint employers who relocated candidates, absolutely yes. A company like Google, Microsoft, Pixar will certainly find plenty of good candidates on Stack Overflow.
In smaller cities without big tech industries, and for people who don't want to relocate, mostly the answer is "not yet" although I expect to get there. The little startup in Tulsa that needs an embedded systems programmer but doesn't want to relocate someone... won't find them. The guy in Oklahoma City who won't move elsewhere isn't finding good tech jobs because there aren't any in Oklahoma City, not because we don't have critical mass.
As for the revenue, earning money was never the point of charging job seekers. We charge job seekers a minimal amount because it eliminates a huge number of unserious, unqualified, and other candidates who really don't stand a chance. If it's free, job seekers have an incentive to post their resume everywhere "just in case" which employers hate, because they have to filter through lots and lot of junk. We're actively looking for better ways to do this kind of pre-filtering. I'd like to lower the cost as much as possible without opening the floodgates, and I imagine we'll find creative ways to let high-rep stackoverflow users get a CV without paying very much. Nothing is set in stone.