$7.87 billion in 2013 / 365 days / 24 hours / 60 minutes / 60 seconds is about $250 a second.
Edit: using more up to date data:
$3,300,000,000 in the 3rd quarter of 2014 / (365/4) / 24 / 60 / 60 = $418.58
Edit: using more up to date data:
$3,300,000,000 in the 3rd quarter of 2014 / (365/4) / 24 / 60 / 60 = $418.58
But considering this is not the peak time in North America, it's should be little less.
When facebook comes up, people will catch up on "lost facebook time" most likely. Over the course of the week (or even day) it will most likely even out.