In most professions, the majority of available positions are not advertised. Instead, they go to insiders of the second degree, i.e., people who know people working at the companies. Employers actually prefer such candidates because they come by way of an existing employee, adding to the trustworthiness and potential fit. Also, hiring without advertising saves considerable amount of money (esp. compared to using a headhunter) — and who, except the average American consumer, doesn’t want to save money in this bad economy?
So if you are looking for a job, try to leverage all your contacts. Make cold calls if possible, but try not to be annoying — so for example, don’t call people at 11 pm, and don’t flood them with inquiries. Be polite, be honest, and be straightforward. But, do not overuse social networking sites, esp. the popular ones like Facebook and LinkedIn. In survey after survey employers express displeasure at employees or candidates using such sites, while these same employers also use these sites to dig "dirt" on potential candidates. (Last but not least, these sites are treasure troves for identity thieves.) Best venues to explore include your academic department, your circle of friends and acquaintances, social clubs you belong to, ethnic clubs you are a member of, your alumni office and network, etc.
Good luck and happy holidays.