How's this for ironic. Facebook, which is having trouble monetizing its vast mobile audience, is experimenting with the old-fashioned postcard to see if can turn a dime.

The giant social network is testing functionality called "Mail A Postcard" in what appears to be a small closed beta that will let users put one of their photos on the front of a postcard, a message on the back, and get the physical address from their friends' profiles or ask them for it.

It'll be stamped with a 32-cent or 45-cent stamp, depending on the size of the postcard, and dropped off at the post office.

For purposes of the beta the testers are restricted to their own photos or their friends' photos.

The widgetry, powered by Sincerely, was reportedly developed during a Facebook Hackathon. The beta testers see a "Mail Postcard" button at the bottom of the photo they're viewing. A click opens an address and message screen. It will cost a fee, but Facebook is supposed to be testing price points.

Sincerely, which raised a $3 million A round 10 months ago, already has an app called Postagram that works with Android and iOS and lets users create postcards out of photos snapped with the phone's camera and send them anywhere in the world for 99 cents. As it happens, Postagram also works with Instagram - hence its name.

"Mail A Postcard" would then appear to fit with Facebook's billion-dollar purchase of the free photo-sharing site, if the regulators ever wave it through.

TechCrunch figures it also fits with Karma, the gift-giving start-up Facebook bought a few months ago.

There are a bunch of postal operators and companies, including Apple, Postcard On The Run and Touchnote, already doing hybrid postcards. Sincerely also has a product called Ink that lets users create and send 5×7-inch holiday postcards personalized using their own photos that start at $1.69 including postage.

It is assumed the close-to-bankrupt United States Postal Service is praying the Facebook widgetry catches on.