Facebook is one of the most useful tools for attracting blog traffic. For those that don't know, Facebook is a social networking site. Anyone over the age of thirteen can have a Facebook account. Users have personal profiles and can post "statuses" that your friends can see in the main area called the "news feed."
Facebook has mutated a few times since it first started. It is now much more blog/business friendly. In addition to your personal profile (which would contain personal information), users can now set up a "page." A page looks like a profile but it is for something other than you. In other words, Alain Gomez would be my personal profile where my friends could see me.
Under Alain Gomez I have started a page for Non-Techie Blogging: https://www.facebook.com/pages/Non-Techie-Blogging/248296061940076. The information you see on a page is strictly about the business. Nothing there is about my personal life but I can choose to interact with people "as" Non-Techie Blogging.
Facebook is useful because it provides a medium for announcing new blog posts to your friends and it allows you to connect with new readers without having to let them see personal family photos.
The key here is not to become spammy. If all you do is use your personal profile to announce blogs, your friends will start to ignore you. Just like how no one wants to hang out with someone that's always trying to sell people their old junk. But sprinkled here in there can provide huge traffic boosts. They are already your friends so they would already be that much more interested in seeing what you have to say on your blog.
A page specifically for your blog allows people to stay current on your latest posts. Unless you have the posts sent to an email account you check regularly, it is easy to forget to keep tabs on your favorite blogs. Most people who use Facebook enjoy browsing through the news feed. If they are following your blog's page, they will see updates for new posts.
Provided that your content is something people want to read, Facebook is an excellent and easy way to keep readers coming back to your blog. Maintaining a healthy blog is all about having a consistent readership.