At last count, more than 30,000 job boards. So competition is fierce. And effectiveness is low. Competition is driving basic transaction prices toward zero. But they try to up-sell value added services, hoping these differentiate them.
Effectiveness is harder to come by, however.
What makes a labor market work is that it consists of lots of smaller micromarkets. Critical mass is when you have enough workers and employers that, when they play, they have a fair shot at making a match. Back to the competition. Big and Small. Big folk, like Monster and HotJobs, are aggregating employer advertising dollars to draw workers.