There seems to be a lot of overlap between what you might put on Facebook and what you might put on a blog. Both can be personal. Both can contain your personal experiences and ideas, what you are doing and what interests you. In other words, both can be like magazines or newspapers.
However, there are some obvious differences in the purpose behind each activity. Facebook is more interactive and the audience is, well, your friends. With a blog, the world of the Web is your potential audience, and your Facebook friends may not even know about or read your blog. Your blog can also be interactive as readers can comment on your posts and you can reply. But again, this interaction would probably be between you and a stranger rather than a friend. So there are, or can be, two distinct audiences.
There may be things you would share with your friends on Facebook that you wouldn’t want your blog audience to know about. On the other hand, you may have thoughts or experiences that you feel more comfortable sharing with strangers on your blog than with your Facebook friends.
Finally, your blog must have a theme, a purpose. People must have a specific reason for wanting to visit and read your blog, be it information, opinion, or business promotion. Your Facebook posts can be random, mundane, spontaneous, it doesn’t really matter because you’re not trying to attract an audience, you’re engaging with friends. So while there is potential for overlap between Facebook and blogging, it may be best to draw boundaries and let your audience be your audience and your friends be your friends.