Landing the right job isn't always about how qualified you are or what you know. Frequently, it's also about how well-connected you are - it's about who you know. If you have a deep professional network, if you have word-of-mouth working for you, if you have the right references, frequently, you'll get much farther than you ever would if you merely had the right qualifications. Working in your favor for the networking part of any job search, social networking sites can be particularly important these days.
The top name in social networking sites for businesses seeking talent and people seeking jobs, is LinkedIn - a place where employers and employees can come together. The job hunter on LinkedIn posts his resume, makes notes about what his goals in a job are, and how he can be contacted. Companies out for the kind of talent he provides can come contact him right away. The more members on LinkedIn a job seeker is in touch with, the more his chances expand of finding the job he wants.