A Facebook user has accidentally discovered an app called Facebook Phonebook. We’re not sure what it is, but it looks like the social network company is testing the app in Canada before a general rollout.

Facebook has been working on a number of mobile technologies and this looks like one of the newest features that it plans to include in its mobile app.

This photo, taken by a friend of VentureBeat editor-in-chief Matt Marshall’s, shows the screen of the app, which appeared on the home screen of the user’s Motorola Droid X Android phone. The app showed up on the user’s phone unsolicited. We’ve pixelated the names and photos of the user’s Facebook friends in the image on the right.

It appears that the app takes the phone numbers from friends that you already have on Facebook and organizes them into a phonebook-like directory. It only lists the friends who have made their phone numbers available on the Facebook pages. You can tap on the friends and then make a phone call to them directly from the Facebook Phonebook app.

Facebook has a Facebook Phonebook feature on its web version that also organizes friends into a directory, as long as they have listed their phone number on their Facebook page and set the privacy setting to be public. It also includes mobile phone numbers. There is a page for a Facebook Phonebook app that was created by third-party developers.

With the Facebook mobile app on an iPhone, you can choose to sync or un-sync your contacts with Facebook. But the weird thing about this app was that it showed up as a separate feature on the user’s DroidX phone, and it was outside of the Facebook mobile app.

We’ve asked Facebook for comment and will update if we hear back from them.

(Thanks to Matt Marshall and friend for the scoop. The photo is grainy since it was taken by a camera phone, and we blurred the names.)