I noticed this problem when my browser's back button didn't work. I investigated and found that when I pressed the back button of my browser during page plugin is embedded on the website displayed, an additional network request was made, and that was to FB. This happens not only on my site but on all sites Page Plugin is embedded, including the official site. This occurs on every browsers, such as FF, Chrome, safari. The error displayed on the page is like "Connection was denied at www.facebook.com". I investigated further more and found that this happens when network security is tightened. For example, when VPN is activated with network security options, I can browse every sites without problems, but webpages where Page Plugin is embedded doesn't work properly.