Alexa doesn't count the internal traffic, like financial transactions. Their website is a tiny tiny share of that traffic. Just think of all the credit card transactions happening all over the world.
1) Facebook is powered by massive infrastructure which can be only be compared to likes of Google. Visa, Mastercard are too small even to be compared.
2) People at Facebook are Smart - as Facebook hired top talent. those "hackers@Facebook" know their sh!t & together they are better prepared that any of us here for thwarting ddos attacks.
so my message to annon is:
Kids, Please don't understand facebook's infrastructure & the smart people who work there.