Hi,
As you can see in my first link, the click on the button generates the error message:
Refused to display
www.facebook.com
in a frame because it is set X-frame-Options to deny
I thought that this message is displayed by the browser because of the CORS settings of your website. And to configure the CORS settings of a Joomla website, you need to modify the settings of the System - HTTP headers plugin of Joomla.
The second link to the PayPal thread was just to illustrate how to modify that plugin.
However, upon replying to you, I looked a bit further on this X-frame-Options error, and there is actually a setting you need to do in your Facebook app to allow your website :
www.youtube.com/watch?v=7CNpLgwa0-c
So it doesn't look like a bug to me. Just a configuration issue.