If a discourse user authenticates with Facebook, does their Facebook user ID get saved in the database?
If so, their profile picture can be found at this URL:
http://graph.facebook.com/USERID/picture?type=small
eg - http://graph.facebook.com/zuck/picture?type=small
edit - or actually, to make it square & 50x50
http://graph.facebook.com/zuck/picture?type=square
(You are supposed to be able to add &height=45&width=45 but it seems that the smallest it will resize to is 48 pixels)
More info here - https://developers.facebook.com/docs/graph-api/reference/v2.0/user/picture