Not Google as previously rumored. Twitch CEO, Emmett Shear confirms in a letter on their site:
Interesting choice. Personally, I think this is a better place for them than Google. Amazon has shown time and time again that they're willing to go all-in on bets and stand by them. Google, on the other hand, acquires companies and they slowly disappear until fully digested by the giant beast that is Google.
Amazon as a content provider is getting more and more interesting.