Hello Domenico,
I have tried to access your site using the given url but it seems like these blogpost aren't public and users need to login first before able to read the full post. Here my screenshot:
http://screencast.com/t/AQJw9rj7toWs . I have made a comparison with the other blogpost which is public:
http://veganbook.biz/index.php?option=com_easyblog&view=entry&id=58&Itemid=636&lang=it and it can shared to Facebook correctly:
http://screencast.com/t/hn7lZFwB2UQ . I believe by making your blogpost public, this issue can be resolved.
To configure this, you need to make the blogpost public by edit
Blog Permission = Viewable by all user as in my screenshot here:
http://screencast.com/t/a1Duw2Mw . If you have anymore question, please don't hesitate to ask us.
Hope this will help.
Thanks.