My business cards have a QR code on the back, so people I meet can just scan it in and it'll add my full contact details into their phone. This means that I can give out a paper business card, and someone can type in my email and number later if they wish, or they can just scan it and give it back to me.

Not for me. I use them, and I take them from other people still.

If you don't have one but the other party does, take theirs and say you'll follow up with an email with your details. It's a nice way to re-ignite the contact.

Another great tip I once got was to write on the back of the card as soon as you get home some of the details of the contact--what event, what date, anything you can remember. A story they told, stuff like that. It helps when you dig up the card later to approach them, you'll remember the conversation better.

But yeah--it varies in the situation or the person you are talking with.

I'm not sure if there's one answer to your question. It also depends on the cultural context: in many parts of Asia business cards are a must.

Andreas