Comment 3 for bug 1316639

It's correct, the application ID is what you should use as ClientId.
I see the facebook site in Italian, so I might not be giving you the exact terms, but let's try:
- in the "advanced" page, you must enable the "native app" switch.
- in the "protection" section within the "advanced" page, enable the "Client OAuth Login" switch
- in the same section, set the "Valid OAuth redirect URIs" to "https://www.facebook.com/connect/login_success.html"

I think that this should be it. Note that one can use other ways to authenticate to facebook (for instance, a different callback URL), if he modifies the .service file accordingly. One can see the /usr/share/accounts/providers/facebook.provider file as an example: the values used there will be used unless different values are specified in the .service file (which has higher priority).