The follow-up in this situation is where the bulk of information should be gained. Starting a conversation can be an easy or a frightening experience. The best way I have found is to either ask an open ended positive question related to the environment or person or make a positive statement about the environment or person. Then follow it up with an open ended question requesting an opinion, feeling, or response from the other person about the environment, an activity, or situation at hand. The rest is easy.
Most business cards end up in the trash.
Many times people exchanging cards believe that the other person is saving their card. I don't do it in a smart manner, I am genuinely interested in their reason for asking. This usually starts the dialogue. Final thoughts - think about all the people who you know that you met through other people. Think about all the benefits you have gained through this matrix of contacts.