I also have an interest in this "Post to Facebook" or "Post to Selected Social Networks" plugin...EXCEPT have one additional ability. Some of the sites have multiple authors (visualize "membership" plugin) which would, of course, have separate FB, Twitter, Instagram, etc. login and fields. Therefore the ability to use social network login data from the membership plugin, or accounts section, or a new section for that matter...would need to be flexible for single or multiple social media accounts from my client's domain.