How important is follow-up with an employer? VERY.
After the Intern Fair, be sure to send a thank you letter to each employer you spoke with.
Yes, this may be time consuming, but it is a way to demonstrate your writing skills, as well as show your knowledge of business etiquette. It will help them remember you as a serious candidate!
Thank them for the time they spent with you at the fair, restate your interest in their position (and restate how your skills and interests match up), and restate your understanding of the next step– applying on-line, or requesting an interview, or completing their application, or waiting to hear from them. Include your understanding of whatever the employer told you is the next step in their hiring process — and then DO IT!
If your conversation was very tentative and you are not planning to apply, you can still thank them in a professional way for their time. You never know if you might be applying to this employer next year, or some other time, and you can establish a strong professional business reputation by demonstrating the courtesy of a well-written thank you letter.