Do these ratings take into account all the folks who just watch every sketch on YouTube on Sunday, Monday, etc.? Because that's ... a lot of folks.
Nope. Especially, late night network TV now tries to track how viral they are instead of their ratings. The ratings for the four weeknight shows are virtually non-existent but they pay these guys; Kimmel, Fallon, Colbert, and Corden for their ability to be viral. SNL is a bit different because it's weekly, they get more ad revenue, but for the most part the ability to go viral is much more important.