Sorry about the trouble. Facebook is frustrating for us too. We don't control anything on Facebook's side, so it's hard for us to troubleshoot problems like this. From our end, everything appears as if it's working smoothly. We don't get any errors back from Facebook. Our best guess is that when you change privacy settings at Facebook, it severs the link with Fuelly for one reason or another. Maybe Facebook wants to have the applications added under you new preferences, I don't know.
The key to resetting your Facebook/Fuelly connection is to remove Fuelly at Facebook and then re-add it. Resetting it on our end won't do anything until you remove the Facebook connection on their end.
So give that a shot. Go to Facebook, Account Settings, Apps, and completely remove Fuelly. Then go back to Fuelly and remove your Facebook connection and re-add it. Then add a test fuel-up to see if it's working.