Oh, quite true, and it may not be the most natural behavior for many of us.

Still, I think most people would understand it if you politely explained that time is limited and you're trying to meet as many new people as possible, and here's my contact info so we can continue this discussion later.

If folks are waiting in line to talk to you it's fine to say "do you mind if I give you a call or drop you an e-mail to do a follow up" and indicate (politely) that you would like to talk to folks who are waiting. Individuals will sometimes come up after a talk and want to have a 15 minute conversation--or try and have an argument of some sort--when three other people are waiting. It's not being rude to spend 30 seconds to a minute to understand the gist of what they want, make a note of it on their card, and follow up later.