Uh, but the programme production companies sell their programmes to those channels.
They know exactly how many people are watching online and they accomodate for it.
The shows don't randomly end up on Netflix or Hulu or iTunes. The producers sell the rights to show their programmes to those companies, and those companies inturn give information back to the producers on how many people are viewing their stuff.
Then there are TV channels that have their own online-services, like BBC iPlayer, who can directly view how many people are watching their stuff, and decide the worth of their programmes from that.
The only thing that TV channels/producers can't entirely accomodate for is videos of their programmes which end up on YouTube or are shared via Pirated Torrents or on illegal streaming websites.
But the majority of views don't come from those, rather from actual Television figures and Online providers.