August 14th, 2016 Update
The ‘ngfb_sharing_url’ filter arguments have changed since this post was first published in 2013. Review the complete NGFB filters list for the latest filter arguments. Also note that NextGEN Facebook (NGFB) has been deprecated and replaced by the WordPress Social Sharing Optimization (WPSSO) plugin since September 14th, 2015.
The main feature of the NGFB plugin for WordPress is to add Open Graph and Twitter Card meta tags (among others) to the head section of webpages, but it also offers social buttons for sharing webpages. The social buttons can be added to the content, as shortcodes, widgets, etc. In version 6.9, a new filter was added for the URL shared by each social button. A “source ID” is passed to the filter along with the URL, to identify the social button needing the URL, and its location (content, shortcode, widget, etc.).