Plug-in changes how Posts appear on site!?

I’m guessing that you have some kind of shortcode or other plugin that is incorrectly outputting code in a filter or shortcode instead of returning it. Try disabling other plugins to see which one is causing the issue.

The plugin works fine for multiple users. Just have them connect their user accounts to their Facebook accounts on their profile page. Or have them enable the publishing box on the Screen Options dropdown on the new post page.