My 70 year old father has a facebook account and doesn't have the slighest clue what do with it, consequently it goes unused.

They may 500 million accounts but they only get 150 million or less users per month. Of that, I bet 50 million of them make up the lion's share of the pageviews. 5 million are complete facebook nutters playing farmville and mobwars 24/7.

If Facebook is a nation of 500 million then the US is a nation of billions, if you count all the tourists and business types flying in/out.