With facebook Username, you can personalize your Facebook URL (web address) by selecting a unique username. It will appear in the location bar of your browser after "http://www.facebook.com/" when you view your profile.
If you are interested in adding the Facebook Share button to your Blog or Website, just go to http://www.facebook.com/share_partners.php . On the page, you will see the codes for Link Only, Link & Image and Styled Link & Image.
Out of the three codes, copy the one of your choice, Edit it by inserting the link you want to Share or the URL of your blog/website. Add the code to your Blog/Web page.