It's worth noting that in the 2 year period since Amazon bought Twitch, their search interest via Google has been stagnant.
https://www.google.com/trends/explore?date=2014-08-25%202016...
https://www.google.com/trends/explore?date=2014-08-25%202016...
The idea is that Twitch viewership is correlated with Google searches - not that Google search is responsible for all Twitch traffic.
Even if a only a small percentage of Twitch users use Google search to access Twitch, the graph would be somewhat indicative of it's growth.
If 90% of new Twitch users reach Twitch over Google, 30% of one week old users and only 5% of one month old users, then a flat graph could actually indicate growth.