When you hit publish, a 'share' button will be at the top, it will have facebook and twitter. click one at a time, a new window will pop up and you may have to long in, then click share (facebook). The twitter will do the same.
If you forget to share your work, go to the bottom of your Hub, you will see a share button as well. Do the same. There are more to choose from on this option if you have other accounts on digg etc...
If as you are reading Hubs, and you find one interesting that you would like your friends on facebook to read, you can 'share' these with them also.