Why does facebook always make you wait to login because of site maintenance?

I am so irritated by this. i try to log on and it says: Your account is currently unavailable due to site maintenance or some sh** like that. WHAT DOES THIS MEAN? I AM SICK AND TIRED OF IT.|||no yaar log in again|||I have been getting this daily for the last week, some days I am unable to log in at all and it takes about 24 hours for me to get back on to the website, its driving me crazy! Hopefully someone can help