First priority in job fair lines is always to read the employer materials. But what then? Time to stare off into space? No. Long lines hold yet another opportunity for you to take advantage of in your job search. More networking.
Use your time productively. Even waiting time can be productive time.
Simply turn to those in front or in back of you and ask them the standard job fair question: "What are you looking for here today?" Ask them about other employers they may have spoken with at the job fair—the good, the bad, and the ugly. Avoid the ugly. And ask them about their job search in general. "Any particularly promising employers?"
Remember that we all like to talk about ourselves. Now is not the time for you to spout on excessively about your success (or lack thereof) on your most recent job search excursion. You are there to listen and gather information. You will learn infinitely more by listening to others than by listening to yourself.
Take copious mental notes. And remember that your network has just grown in size by one.