Most hiring decisions are made through the informal networks. That has, historically, been one of the great benefits of going to a really good school, because you get to make weak ties with folks who will eventually probably be in a position to help you out (and vice versa). Participation online is a staggeringly good way to make weak ties.
Meanwhile, the "Send out a hundred resumes and then wait by the phone" is jobseeking for the truly clueless (and, sadly relatedly, the entry level -- I swear, there should be a class on this at college). It virtually guarantees that any company putting up a job will get a hundred resumes for it, most of them from people who have no capability to actually do the job. Additionally, all the people who are poor candidates on day N will probably still need jobs on day N+1, and they'll probably still be sending resumes. It doesn't take a genius to predict what this does to your applicant pool after iterating for a while.
I think if you're really good your jobseeking process is less likely to involve seeking out companies and more likely to involve saying "I am available for work" in a semi-public fashion, but that is another discussion altogether.