WhatsApp? Is this so difficult to program? Just VOIP and messaging? The Chinese WeChat hat much for features (e.g. send money, pay etc.)
And even my mother has installed Threema on her phone because a friend of her asked for it. This tells you something. It is hard to come up with another social network like facebook. But a chatting client on the phone? The tide can change very fast there.
Threema gets used more in Europe than in the states. But even a US buddy recently suggested we switch to threema. It is closed source but claims to offer more privacy.