On any given block in Buenos Aires, you are likely to see someone speaking into their phone, but not on it; talking to someone, but not necessarily with anyone. I recently visited the city, and was struck by the fact that it seemed like all the citizens were walking around expressively talking to themselves. In reality, most people are perpetually sending voice memos to one another.
WhatsApp voice memo usage has skyrocketed in the past year among my circle of friends. Interestingly, even if some of them believe iMessage is an overall superior app, they stick with WhatsApp because it works for everyone (Android friends and even a few of them on Windows Phone). A day doesn't go by without at least 50 WhatsApp voice memos in busy group threads.