Do all the heavy lifting facebook stuff on the backend. The only thing the Roku needs to do is pass user identification via http POST/GET.
Example:
The user clicks on the "Share to Facebook" button.
The Roku contacts
http://www.yourserver.com/facebook.php?id=12345678The backend server replies "I don't know you. Please visit
www.yourserver.com/facebook to link your facebook account" or "I know you, post whatever it is to facebook".
Its a lot easier than trying to make the Roku do everything.