I'd say this: state's aren't looking for content. They're looking for behavior. Social-graphs and related metadata is far more revealing than reading the contents of messages. For targeted espionage by human analysts matrix has a nice function to replicate database silently to a third-party (without the user's knowledge). Though that's not really needed (end-points are wide open and vulnerable), but it's there as a core of the protocol.

Again, encryption works at obfuscating the message. Mass surveillance at a state level doesn't use the contents, but has developed other methods. The contents themselves are irrelevant. The man-power to create a surveillance regime on content makes it infeasible.