TechCrunch has come across some yet-unannounced features, thanks to a Stanford computer science student:
Messenger’s payment option lets users send money in a message similar to how they can send a photo. Users can add a debit card in Messenger, or use one they already have on file with Facebook. An in-app pincode also exists for added security around payments.
Check out the article. They've got screenshots, too. Could the long-rumoured Bitcoin integration play into this as well?