As any career coach can tell you, networking is hands-down the best way to find a job. The problem is, it takes a long time to develop a network. (If you wait until you're out of a job, it's too late.) This applies to online networking as well.
While other web-based networking tools exist, I like LinkedIn. While it is specifically for business networking, it's not industry-specific, and it has all the tools & features an online networking tool should have. You can build a pretty significant network using LinkedIn, which can come in handy in many ways -- including just keeping in touch with colleagues.
I mention it here because it can be an efficient, powerful way to support a job search. Just keeping your professional network apprised of your job hunting progress can help (they can keep an eye out for suitable opportunities, for example).
Again, don't wait until you need a healthy, substantial network to start putting one together. LinkedIn is free. Why wait?