Sourced from an early postach.io post - http://blog.postach.io/new-embed-support-for-twitter-vimeo-youtube-about-me-and-more

Posting a notice to Facebook about your latest post. Note that you need to connect to Facebook in the Edit Settings section of your site.

IMPORTANT

If you tick this box, then a notification will go to Facebook every time you post, and not only when the 'fb' tag is set.