First off, for those who've never been to one, a job fair usually involves a number of companies setting up tables, in front of which, hopeful job seekers will line up in order to get a chance to talk with an HR rep who may or may not have positions to fill. The organizers may also have set up refreshments (which leads to mixer-style networking) and possibly some educational workshops or seminars. Let's ignore that other stuff for now and focus on how you connect with the company representative who's sitting behind the table.
In short, you can't.
That person, no matter how nice they are, for as long as they are sitting at that table, they are there as the embodiment of their company. As such they will tend to think of the people waiting in line as "potential employees" and not as "Bob", "Larry", and "Jennifer". So, is the job fair a waste of time as far as networking goes? No, but it takes a slightly different mindset. Here are a few ideas that might help it pay off.
- Connect in line. While you probably won't connect with the people behind the table (unless they are exceptional networkers themselves). You can connect with the other folks standing in line with you. Share tips, techniques, and war stories. Basically treat this as an impromptu one-to-one. Ask for their advice. Everyone loves to feel like an expert. You may walk away with some good ideas and a few new connections.
- Be an expert. If you have some skill that would be beneficial for other job seekers to know about (resume/cover letter writing, networking, business card design, meditation, stress reduction, time management, financial management, etc), use it as a mechanism to connect with the organizers. Find out who they are and offer to help by giving a short workshop. Organizers are often looking for speakers who are willing to volunteer their time. Even if they aren't interested, you at least get to stand out as someone who is willing to help.
- Volunteer. If you don't have expertise that applies, you can still offer to help. One thing you can do is offer to show up early to help the company reps set up and get organized. This is a good time to meet them as a person. Since you are helping them and not trying to get something from them, they are more likely to see you as a person, too.
Of course, you may still find success talking with the representative after waiting in line. I'm guessing, though, that meeting them as a person first couldn't hurt the process. Just keep in mind that networking is always about the other person. Make the interaction be about them and see where things go from there.