Once you’re on the job market, it’s usually too late to build a network. Of course, you should still make every effort, but networking from a position of need makes it harder to make real connections.
If you’re already unemployed and looking, anyone you meet will see what they can do for you, but they won’t know why knowing you benefits them. That’s understandable, since at that point your only goal in building a network would be to help yourself.
In reality, it’s best to make networking part of what you do at all