Well, I believe you can't get this until you are the owner of this post. If you were the one who created this post on facebook or one of the admins of the page which is linked to (http://www.facebook.com/events/385623724876261
) event then you would get the shares count.
Try fetching the information from the admin's access token or the one who created this event. If that doesn't work out for then the last thing you can do is try fetching this piece of information from page access token (which belongs to a page). Let me know if you are still not able to get this piece of information even after trying with Post Owner's token, Admin's token and lastly Page Access token.
Cheers,
Reno Jones