Short answer
Add &status=0 to your js.src URL to make the warning disappear, ie:
//connect.facebook.net/fr_FR/all.js#xfbml=1&status=0
Full answer
FB.init() will be called internally by the facebook script upon loading if parameters are provided after the hash (#) sign. Here xfbml is passed, so FB.init() is called.
(source code: http://connect.facebook.net/fr_CA/all/debug.js line 8699 at the time of this post)
The default args for init() are used if not explicitly provided: status arg default is true – which makes the FB script call getLoginStatus() at startup, which complains because an app ID is needed for that function call.
FB social plugins doesn’t need an app ID – they are rendered into an iframe originating from facebook.com, so FB login status and cookies as accessible to them.
The “Get the Code” wizards in the FB developers Social Plug-ins section generates a URL with the xfbml param, it should be updated with the status=0 param IMHO.