How to I authenticate a logged in Facebook user calling a Meteor API?


Here is my use case - I have FB login working for my Meteor app. I am now writing a FB app in which I want to call functions in my Meteor server (probably the easiest way to do this is by adding Rest package?). e.g. get a users shopping list. How can this external call be authenticated inside Meteor to get the correct userId? From FB I have the senderId of the user.