As well as promoting the directory, one of the purposes of the facebook page is to include a link to every newly added blog to the directory. This will, combined with the new layout of the category pages, mean that each new listing will get up to 15 quality and relevant back links to their blog.
Using Facebook is also a way of spreading word about the blog directory, especially to those who do not use twitter. The more people notice the blog and list/visit then the more your backlinks become worth!