And it may only take a secondary use case to reach critical mass. People I know who use WhatsApp seem to have started using it to avoid per-text charges, not to get end-to-end encrypted messaging. But now, even if Facebook's estimate [2] of 450 million WhatsApp users is 90% inflated, there are 45 million people using encrypted texting, which I would not have predicted.
I think the number is much higher than that due to Messsages+iCloud usage by iPhone and other Apple products also constituting end-to-end encrypted text. Yep… Just a few years ago, nobody cared about end-to-end encrypted text, today, still most people don’t know or care what it is, but I bet there are enough people using it without even realizing to constitute “most” or something very close to it. (Between Skype, WhatsApp, Messages/iMessage, and others). Owen