A select group of Facebook users — maybe even you! — can now find a new option when looking at photos called "mail a postcard." Once the desired photo is selected, it becomes one side of the card, and you have the ability to enter personalized text on the other. You can then address it from memory or use an option to ask your recipients for their physical mailing addresses.
Sending postcards this way isn't free, but Facebook hasn't decided exactly how much the feature will cost when it's finally rolled out to all of its users. It's reportedly testing a variety of price points now, so hopefully we'll hear more about this novel feature soon.