Based on their own contact us page. They are selling candidate & job listing services. Just think about how many people are pouring their knowledge into stack exchange based services. All these people have accounts and all these people have their information saved into stack exchange. So if any of us go looking for a job or are open to being approached by head hunters and a like well stack exchange can connect us.
This model is much better than say monster.ca or other CV services as we are put to the test asking and answering questions that potetnial employers can see and judge us on. Of course traditional non flashy advertising as well.
3.CV database search
4.Probably others I can't see from the outside looking in.