there is a section at the bottom of your hub that says 'share this',
you click on the box and you will see facebook as a link. click on facebook and it links your hub to your profile page. you have the option to write something in the comment box when facebook opens its link box.
also, right after you click publish when you have finished a hub, there is a section at the top of page that asks you if you want to link to facebook, click yes. it will post your freshly published hub!!
hope this helps.