If you’re running an up-to-date Discourse instance you should now be able to add a token that will allow oneboxes to be generated using the new API.
I’ve not yet written up step-by-step instructions on how to generate the token, but if you are comfortable in Facebook’s developer site you can go through the ‘Requirements’ listed here:
to generate a token. That token can then be entered as the value for the facebook_app_access_token SiteSetting.