Take it from Upworthy’s top curator, Adam Mordecai:
Rarely, if ever, does content truly go viral ... I think 0.5% of everything we do goes beyond one million views.
Addendum (12/31/2013): Put another way (again from Mordecai):
Anyone who says they can make anything go viral is probably a snake oil salesman or really naive. Most of our stuff fails.
Addendum (1/8/2014): One more Mordecai slide: