hi there !
I faced the same problem and found a fix.
After anylising how fb handles youtube video with open graph, i found that the og:video meta was using a wrong url style http://www.youtube.com/embed/*** where it should be http://www.youtube.com/v/***
So I added this :
$og_video['og:video'] = 'http://www.youtube.com/v/'.$match.'';
after line 1013 of nextgen-facebook.php which contains :
$og_video['og:image'] = 'http://img.youtube.com/vi/'.$match.'/0.jpg';
now when sharing a post with a video inside it works properly and the video can be played on embed yt player on facebook.
I'm sure this is not a clean way of doing it and that you'll find a better way of renaming the yt video url for it to get properly embed by facebook on posts.
do you think you could add this fix to the plugin so it would work in the future in case of updates ?
thanks a lot for the GREAT WP plugin ;-)