I would like to know if anyone knows any app|plugin|way that I can connect the comments under a specific post in fb with fb-comment-plugin under a specific webpage. For example anyone who comments on a webpage with facebook comments plugin becomes visible under a specific post in a fb page and vise versa.
This is not possible, the Comments Plugin can´t show the comments of a page post (or album/photo/whatever) on facebook. You can also try this in the Plugin Configurator: https://developers.facebook.com/docs/reference/plugins/comments/
You have to connect a specific URL to the Plugin, but it does not work with a facebook.com URL. Nothing will be shown.