We've all been there. You're at a professional or social event and have a good conversation with someone. You want to keep in touch, so you offer your business card (if you have one) before parting ways, and the other person does the same. If you don't have one to give out, you may simply ask for theirs and assure them you'll follow up with an email with your contact information. Note: make sure you actually do this the following day.