One of the core tenets of the Netflix culture is "Freedom & Responsibility". In this context it means that each team is free to make calls like that as they see fit. Promises, Observables, Generators, Callbacks -- it's up to your team to determine what makes the most sense.

That said, we do make heavy use of Rx on the API layer (RxJava/RxGroovy). My team isn't currently using RxJs, but I expect that to change in the future. There are a lot of Observable fans here :)